Keyboards and drumkit.
For some reason those defeat my ability to coordinate my limbs. On piano I can just about whack out easy chords with the left hand while playing a laughably simple melody with the right, but I can't get enough of a handle on it to make the exercise worthwhile.
A kit of drums not only tries to make me coordinate four limbs (ain't going to happen), but a kit is big and cumbersome. For someone used to carrying mandolins, fiddles and tin whistles, a large instrument doesn't appeal come packing up time.
Uillean pipes, great highland pipes... actually, ANY kind of bagpipe. I would love to, but life's too short and pipes too pricey. Years ago, the rocker in me would have laughed the idea of bagpipes out of court. Liam O'Flynn, Finbar Furey, Katherine Tickell and the fella out of Tannahill Weavers soon converted me.