My dad just emailed me and gave me a few suggestions of things that my grandfather did with my sister and I when he found himself on baby sitting duty:
1. Play hide and seek with him in living room and count to 100 - several times (he used to watch and think "they will fall asleep in their hiding places before he moved - then what, how will we ever find them").
2. Have them work on a jig saw puzzle (but remove 10% of the pieces - every 30 minutes or so drop one near the table).
3. Scavenger hunt (again removing several items until they are an hour into it then selectively place one or two items back into play).
I know all this is sneaky, but I prefer to call it clever or creative stress management allocation.