Do very simple crafts, things that can be done by the children alone if they don't have a grandparent there. I always just tell the children w/o grandparents (ahead of time) that they are making a craft
for their grandparents and they can take/send it to them.
Decorating things --cards, photo frames, placemats, etc-- is fun and easy and everyone can be as creative as they wish. You can buy inexpensive tubs of foam shapes or have diecut shapes available.
Don't do anything too messy and be very careful of the types of crafts they have at places like Oriental Trading--they have so many little pieces! Great for small groups, a nightmare for large groups (believe me, this comes from personal Grandparents Day experience

)