I use small dog/cat carriers for my rabbits. I have food and water cups that clip into the door, bird coop cups work well too if you can't find anything else. A small towel works well in the bottom. I also put hay in for them to munch on. A larger (medium size cat carrier) can hold a corner litter box as well.
I would not use a carrier that is too big as it can become dangerous in an accident. Enough room to sit up and turn around should be good. A bigger rabbit or a pair will need a bigger carrier.
Hard sided is better as soft ones can be easily chewed. Hard ones do protect in an accident. If needed, you can stack them, but you do need to make sure they won't be moving around once you get moving.
I would keep the rabbits in the car with you. You can better monitor them and keep the temperature comfortable.
If you are staying in a hotel, make sure they allow pets and even specifically rabbits. Bring a pen or cage so your rabbits can get out. If you use a pen, have a tarp or some plastic to protect the carpet.
Bring lots of food and have it easy to access. You will want at least enough for the trip and then some for a few day when you get there and unpack. Having at least a full bag will allow you time to get settled and be able to transition food if you have to. If you have a lot of rabbits, then at least a couple weeks worth of food is needed. The same goes for hay, bring enough for the trip and extra for when you get there. Veggies you can usually buy on the way so they are fresh. IT can be good to bring water or use bottled water as it can be different from place to place and you want your rabbits to keep drinking.
If the trip will be over a couple days, you will want to bring extra bedding for the carrier. 1 towel per day should be enough. Puppy pads can also be used if your rabbit won't chew them, they don't take up much room and can be thrown out after.
Since you have time before you travel, start getting your rabbits used to travel now. Buy the carrier and have them in your rabbits area. You can put treats inside to encourage them to go in (leave the door off so it doens't get in the way). You can also go on outings so they get used to going out and coming back, pet stores are good for this as they are usually pet friendly. Even just driving around in the car for a bit can help get them used to the travel. There isn't too much you can do to prepare them for the full journey, but having them be comfortable in the carrier and car helps a lot.