There are lots of flatwork options out there. I do large figure 8's, "hourglasses" across the diagonal, 3 and 5 loop serpentines, ride the rail and do a circle in each corner, sometimes I just do random "squigglies": loops and circles. Figure 8 shortways (ride up the centerline as the touching-center-point of your 8 ). Riding squares can help switch it up, too.
Leg yield from the quarter line to the rail and back again, lots of transitions within each figure.
Good old fashioned trail rides, long or short, do wonders for bored horses. Even riding around the rail outside
the arena can get them interested again.