I’ve skated for a few thousand km and this is what my gear list has boiled down to:

Skating gear

• The longboard itself, and I’m not going to give you any advice because you already have that.
• A helmet. I prefer using cycling helmets as they have better ventilation, regular skate helmets aren’t designed for that. You can put extra reflectors on the back of the helmet.
• Lights. Don’t skate in the dark without lights. There are two options: you can get a helmet-mounted flashlight (I’m using the ridiculously overpriced Cateye Volt 400 Duplex) or you can get a helmet with an integrated light: Rockbros has a great helmet (available at AliExpress) with a light that is even more powerful than my Cateye, plus extra blue and red lights on the sides and on the back. Why do I use the separate light? I’m a bit nervous about the integrated light compromising the safety of the helmet, perhaps there’s a reason only Chinese helmets have that feature. Another thing is that it’s nice to have a lighter helmet during the day, you only put the light on it as it’s getting dark. It makes a great difference on a bicycle and is less important on a longboard (due to the different stance) but still that means taking a few extra grams off your neck.
• Safety equipment. A blinking light on the backpack and perhaps some reflective strips to put on your legs. Don’t be afraid of looking like a Christmas tree, there’s no such thing as too much safety. Also, speaking of other kinds of safety, some pepper spray may be a good idea (depending on the country), and a reflective emergency blanket weights nothing and may save your life.
• A skate tool (I prefer the metal Paris tool, it’s compact and reliable) and bearing lube (I use Skanunu BCL).
• Shoes, this is kinda individual and I guess most people will find my choice weird but I skate in ASICS Patriot running shoes. They are dirt cheap, quite durable, very light and flexible, and they boast good ventilation. They won’t fit people who like stiff shoes though.

Bags

• A good backpack is extremely important. The first thing you want to consider is size. Do you need a big backpack? If you aren’t going to camp outdoors the answer is NO. I skated for a month straight with a 15 L backpack and it was big enough. There’s a 17 L backpack that is perfect for longboard trips, and that’s Deuter Race EXP Air. It has amazing snug fit (you can even run with it), a metal frame and a separate mesh back for better ventilation, a bright acid green reflective rain cover (which I recommend using all the time, even thought the backpack itself can withstand a moderate rain – it’s just very visible on the road), a retractable helmet net (which you can use for carrying your lunch), one big compartment and one flat compartment. I’ve upgraded to it from the old 15 L version which had served me for many years and still was in a good condition (gave it away to a friend who’s still using it). If you want to save money Decathlon has some cheaper alternatives I believe, but I don’t know whether they’re good.
• Storage bags. I use a small Acteon cube for underwear and socks and a big one for t-shirts and shorts (optional). Actually you can replace it with a cheaper bag as you won’t have enough clothes for the packing cube to be useful. Just search AliExpress for “ultralight waterproof storage bag” and you’ll find a variety of cheap bags. One will house your underwear and socks, one will house your toiletries (I’m biased against toiletry bags as they are usually unnecessarily big and heavy), one will house your charger and cables… and that’s all, you are unlikely to need more.
• A wallet. I use a Paper Wallet and it’s amazing, just big enough for a card and some cash. Don’t forget to store a second card and some backup cash deep in your backpack though.

Toiletries

• A foldable toothbrush. The ones I used got old and I can’t find a worthy replacement though. I have a POLO but it’s kinda disappointing and the ones I got from AliExpress sucked. Pack some toothpaste as well, even if the hotel has some it usually sucks.
• A 60 ml silicone bottle with shampoo and shower gel (2 in 1). Incredibly convenient, easily refillable. Guess where I got mine? Yep, AliExpress.
• A travel towel. I use a Naturehike microfiber towel (40×80) which is very compact and light, yet big enough for me to get dry, though you may want to buy a bigger one if you have long hair.
• A small roll of toilet paper in a plastic bag. You will be thankful one day.
• Nail clippers if you’re planning a longer trip.
• A box with a piece of soap. You will use it for washing clothes in the hotel sink, or just for washing your hands (many hotels don’t provide soap).
• Optional: a small hairbrush, a razor (I just let my beard grow), some Melatonin pills (I used 2 or 3 of them in a month when I had problems sleeping because good sleep is vital, 3 mg was more than enough for me), a bottle of sunscreen (believe me, you don’t need sunburns when traveling).

Clothes

• You only really need two pieces of underwear, and in my opinion Uniqlo AIRism boxers nail it. As soon as you get to the hotel, you wash the boxers you were wearing, take a shower and put the other piece on. The wet ones will be dry by the morning. However you may take one or two extra pieces for those lazy days.
• Two or three pairs of socks will be sufficient.
• If you’re traveling in hot weather (it was +42º in Daegu when I skated across Korea) you only need running or cycling shorts and one t-shirt. Again, you are supposed to wash your clothes as soon as you get to the hotel.
• Cycling sleeves will protect your arms from sunburns and somehow make it seem cooler. They are very light and compact. Must have when traveling in summer.
• Colder weather complicates things a bit. I prefer using layers, that allows me to skate even when the temperature drops below 10º. First this longsleeve, then a t-shirt, then a warm longsleeve, then a jacket. Almost everything I wear in a trip in colder weather is from Decathlon’s winter running gear list, it’s good and relatively cheap. You may also want to use a beanie when it gets cold, just put it under the helmet and you’re warm again.
• Speaking of a jacket, this is a great option.

Tech

• Your smartphone is vital. You may even want to have a cheap backup phone somewhere in your backpack. Don’t take tablets or laptops (I did once and that was a mistake).
• A powerbank, 10 Ah is the perfect size. Make sure it has at least two USB ports.
• A charger with at least three USB ports, I use either a cheap Chinese 3-port charger or Zendure Passport. Always charge all your tech, especially the powerbank. Take enough cables. Keep one at hand in case you’ll have to charge your phone on the way.
• If you want to shoot videos, a dedicated video camera like DJI Osmo Pocket will be so much better than a phone. You can use a phone gimbal of course but it’s really inconvenient – believe me, I’ve tried it.
• Optional: a BT speaker (I put my JBL Clip 3 on my chest for navigation, calls and music), a heart rate sensor (be careful with your heart and don’t skate with high heart rate for prolonged time; you can use an expensive brand model or a cheap Chinese one, they are more or less the same in terms of accuracy), a sports watch (Garmin Fenix watches are the best ones ever, you can buy a used Fenix 3 to save money, or get a cheaper Garmin, but don’t waste your money on Google Wear watches or Apple Watch, they won’t last long enough), it will be convenient for controlling your heart rate and for scheduling switch. If you want to make music on the way, you can pack a portable synth like a Pocket Operator or the OP-Z but in my experience I had no energy for music after skating for the whole day.

You should be easily able to fit it all in the backpack (and don’t forget your passport), and it will be 4-6 kg without water. If it’s over 6 kg or if it doesn’t fit, reconsider your equipment, you may conclude you don’t need some of the things you tried to cram in it.