I often times think about the concertina, but in the end I convince myself that I still have a lot to learn on the ukulele and it is probably better that I don't try to divide what time I have between two instruments. I'm really afraid that once I started on the concertina I would not play my ukulele anymore.
I do also play a hand drum, but it is specific to Plena music. I've been doing that for five years. Plena is a Puerto Rican folk music. There is generally a group that is the nucleus, but roving Plenaros often times come along and join in. On the weekends in parts of the island you will hear bands playing and migrate to them with your drum. In Plena there are three drums, each with its own beat. Together the three sound pretty cool, but individually they don't do much. The trick is for the three drums to stay in rhythm with each other. There are lots of stops and starts that have to be right on the beat. If one gets off, it takes everything down with it and the offender is told to leave. Not for the faint of heart, getting publicly ejected I mean. You pretty much practice by playing with the other two drums, it is an OJT thing, so not something you sit in your basement and do. To learn Plena, they teach you the beat for one of the drums. Then they pair you up with an experienced player. You watch the hands of the person you are paired with and use them as your guide until you learn to feel the rhythm. If you get lost or off, it isn't all that bad really, the leader will nod at you, call you out and give you the stink eye if you are not paying attention, and then you sit out for a while and try to get a feel for what everyone else is doing and how you fit into it. Then you do a couple of years where you are an apprentice and you just learn to keep the beat and don't do anything fancy. Then after a few years you really get to feel the music and start doing a little free style within your beat. You know that you've made it when the leader starts calling you up and telling you where they are playing. Kind of like you get a personal invitation and you're a part of the nuclear group. No more roving. But I love playing Plena and play every weekend in PR.