I would suggest a portable steel drum-the kind cut at all different angles to get differnt notes...fun instrument, interesting sound,,,you can do straight up melodies or use 2 (or more!) hammers per hand if you want to get some chords in there. I think Lark s in the morning music (There's a shop in Seattle and they have a catalogue) has them for a good price...or they did a few years back. You can set it on a stand or hang it around your neck.
One word of caution-make sure to carry it safely, a dent will change the sound forever.

So..is there a limit on the number of instruments one person can learn?   (I knew I should've stopped taking recorder lessons in 4th grade)