Hand sewing, including English paper piecing. There are quite a few quilters who slowly work on these hand projects when they travel or don't have access to their machine.
This. I've recently started quilting (by hand). If you do baby quilts, you can get buy with needles, safety pins (to baste things in place), thread (one for piecing, one for quilting), scissors or a rotary cutter (if using the rotary cutter, you'll want a cutting mat, as big or as small as you like), thimble(s)-- I like leather with a "coin" for piecing, a metal one for quilting, and your batting/fabric. I also use a lap hoop stand and an "Aunt Becky"; a little piece of folded metal to protect my underside thumb/fingers from needle pricks.
I wanted a portable hobby, to take with me for any "found moments". I have a 14" lap stand quilting hoop that fits in a bag, quilt included, with a pocket for my thread and sewing items (which fit in an eyeglass case to keep together). Eye glass case holds extra needles, thimbles, Aunt Becky, and folding scissors. Sometimes I move the thread to a bobbin to make it more portable, fitting into the eye glasses case as well (just remember to thread from the cut end rather than the free end). When I get close to the edges, I don't need the hoop. I actually don't need the hoop at all, but I prefer quilting with it rather than just lap quilting.
I'll need a little bit bigger bag when I move on to queen sized quilts, but I'll make that one. The piecing can all be carried in a smaller bag though, the larger bag not needed until I put the quilt on the hoop.
Crochet is also very portable. A spool of yarn, hook, and folding scissors and you're good to go. Oh, and your pattern if you're following one.