If you're in the hunt, a pinch runner or 2, a short stamina pitcher (e.g. a y with ctr/vsL over 80 but sta at 5), additional d-specialists, and hitters with huge platoon splits are useful callups as well.
As for the arb clock, if you plan to start him at the ML level next year, I wouldn't worry about it. But if you plan to do a 20-day callup and save a pre-arb year, this will change the math on doing that.