Basically you could still play most brass-section instruments like trumpet and baritone. Most actually have 3-4 valves, but variations with 2 exist (typically they're in the key of G, rather than the much more common B flat of 3-4 valve instruments, but that could be worked around). Trombone, which uses a slide and no keys, would work just as well with two fingers.
Woodwinds typically have far more keys and would need to be majorly rethought.
Finger-based keyboard playing (piano) would be very different of course. Still doable but music would have to be much simpler and/or slower.
Mallet-based keyboard (marimba, xylophone) would require a new grip to be invented, and the two-mallets-in-each hand technique some players now use might be impossible, but for the most part wouldn't change. Same for most percussion.
Plucked string instruments (stand-up bass): the answer is the same as finger-based keyboard. Bowed string instruments (violin): probably also the same as finger-based keyboard (because although the primary hand just has to hold a tool, as in mallet-based keyboard, the other hand still has to work frets).