For in the car, a hard cat carrier will work. A small wire cage will also work if you have a large rabbit or pair.
For when you are there, an x-pen or wire dog crate will work. Both fold up easily and are not too heavy. An x-pen will require something to protect the floor, such as a small tarp. Both can fit a litter box, dishes, toys and give space to move around. If your rabbit is a jumper, you should get a taller pen or the crate as it has a top.
What I have done when staying in hotels is to lay down a tarp, then a towel and fleece and put the pen up so the fleece goes past the edge. This makes it easy to clean up and protects the floor.
Depending on how long your are staying and what facilities you have, you should take at least 2 sets of bedding. Having a washer is ideal, but you can at least rinse out towels and fleece and hang them up to dry. Puppy pads can be good under fleece, they don't take up much space and can be thrown out after.