I always have a travel hand project ready to go. Either an English Paper Piece project or a wool appliqué. I love the EPP projects best for car travel. I prep pieces ahead of time all I need is needle, thread, small scissors and my paper basted pieces. I can get a lot sewn together. It's easy to carry, easy to pick up, put down.
Wool appliqué I usually work on in hotels, one in a while I work on them while traveling, but I need more threads generally.