m4 allen head in various lengths up to 50mm are always useful
M8 would be good as well, but they get a lot pricier for larger sizes so probably not cost effective to buy hundreds of them.
This is getting quite specialist - but camera tripods have imperial sizes. If people need to mount a camera on something (bike, skateboard, kite etc) then allen head bolts in these following sizes are really useful and cheaper than buying a special camera mount.
1/4 inch with a thread count of 20 threads per inch (1 inch and 2 inch lengths)
3/8 inch with a thread count of 16 threads per inch (1 inch and 2 inch lengths)
They are normally referred to as "1/4-20" and "3/8-16".