Well, no stirrups is an obvious one, riding in a forward seat/light balanced seat with jumping length stirrups, riding one handed (with or without stirrups). All can be made more difficult, and more interesting, by riding shapes rather than just bouncing round the school - try one handed figures of eights; circles, pole work and changes of rein in the forward seat etc. Beneficial for pretty much any level of rider IMO.
Riding in some sort of formation - side by side, going in opposite directions, etc - good for cheeky RS horses who just want to get to the back of the ride! There's all sorts you can do and I remember always finding it quite tricky, but enjoying it, as a kid. You can make it as easy or as hard as you want - we used to do a direct transition from trot to halt, a turn on the forehand to change the rein then straight back into trot again, all in unison with a rider at each letter marker. Bloomin impossible lol!
'Fun' lessons - gymkhana races, bending in and out of obstacles etc, are always good. We once attempted 'barrel racing' in a riding lesson!
Setting up two poles about six canter strides apart, then trying to get seven, or five strides in - fun for more confident riders as it gives them an excuse to go a bit faster
I'm sure others can think of some much better ideas!
A good test of planning, control and accuracy is this one:
Everyone in single file on one rein in walk
Rear rider comes out of file and trots past the ride
Joins the track ahead of ride and picks up canter in first avaliable place (corner or straight away for more advanced)
Come back to trot before crashing into the ride
trot past the ride again and take up lead file without everyone else crashing into the back!
My instructor uses this one as an assesment exersize as it shows absolutly everything and can be used for every ability as you can be as much of a perfectionist as you wish and for tots its just an acomplishment to get thier pony out of single file! Can also be done with ride at trot (canter past ride) but must have resonably behaved horses! Oh another variation is the rear file halts before a direct transition to trot then past the ride as getting a horse to halt propperly from the rear can be surprisingly hard!