olddude- I keep a bottle of Advil and tube of Voltaren in my case at all times.
Depending on the instrument, besides the things already mentioned:
Autoharp or hammered dulcimer - a tuning wrench and a paint brush (for dusting under the strings)
banjo - wrench for tightening head and a pair of socks (red) to stuff under the dowel stick or rods to mute the sound (I decided to avoid banjo jokes here)
bodhran - spray bottle for dry days, hair dryer for humid days. (I decided to avoid the bodhran jokes here)