The obvious solution is a multipad, for example: roland spd-20, spd-30, spd-sx, alesis samplepad or performance pad, and there are many others on the market. Any multipad should output midi notes triggered when playing the pad but those multipads may not be very "visitor" friendly and are quite hard to integrate into an art installation.
Another solution is to use drum pads (like roland pd-8 or bt-1, alesis dmpad, or any other drum pad on the market) combined with a trigger-to-midi module (like roland tmc-6, alesis trigger-io, ...). in your case, single-zone pads should suffice.
In any case, the pad in itself is just a package around a piezzo-electric sensor ( http://mambohead.com/wp-content/uploads/2011/07/PiezoElements.jpg
). You can wire the piezzo sensor directly to a pad input of any trigger-to-midi module specified above. A piezzo sensor is very cheap (0.50 euro), is easy to glue to the back of any surface with a piece of double-sided tape, and is extremely sensitive so beware of false triggers from the surroundings (trigger-to-midi modules have threshold settings to avoid false triggers).
A piezzo sensor has a polarity, and each manufacturer has its own way of connecting a piezzo to a module. So, careful when mixing pads and modules from multiple manufacturers, although generally reversing the polarity of the pad is enough to make it work.