Zip ties and gaffers tape.
A blue banner
Wood screws and fender washers. Never have 'em, always end up needing them.
I got tired of that same problem and we started making one of these. Contains pretty much every tool needed to hit any fastener on the robot. It’s different every year. The Pit Viper, a pit crew essential - #32 by hpeyerl
One thing I haven’t seen anyone mention yet is seating. A few folding camping chairs for your pit makes a huge difference when you’re in there for 12+ hours a day. Especially true for all of the mentors. Prevents sitting on and breaking random items in the pit, and can be folded away in a corner if you need to make room for serious repairs.
This year we ordered 20 of each of our 4 most common wrench sizes from msc direct (bondhus brand) and used paint markers to color code them.
Deodorant. Not just limited to my team.