I was actively involved with a lot of care for my three grands from the time they were 2, 4, and 5 until 6, 8, and 9. They were in day care or school part of the time and with me for part of the time. I could not possibly have provided all the enrichment they got at day care--books, toys, movies, activities, field trips, other kids--the list goes on and on. At my house they tended to have a little too much TV time, because I have cable and their mother didn't. This was a tough one--they really, really, really wanted to zone out and watch their shows--and by the time I picked them up from daycare or school, they were tired, and really needed a bit of a break. We did do quite a bit of art, and also baking and decorating seasonal cakes and cookies. They practiced their piano at Grandma's. (All three were in lessons from age 3.) They didn't take much interest in the natural sciences--I lived across the road from a beautiful park, and taking walks, identifying birds and trees, etc. was something I loved and they didn't. I was moderately successful in getting them interested in identifying stars, constellations, and watching for the space station to go over. (In the winter, when it was dark early.) I started with the kids at 63, and ended at 67 when my daughter moved away and so did I, so daily childcare was no longer feasible. There was no impact on my health whatsoever--I could keep up with them fine--and I don't nap.
One vital point I have not seen mentioned: Mom's rules, Dad's rules, and Grandma's rules all need to be the same. Adults should figure this out somewhere away from the kids--then everybody's expectations will be the same...no confusion for the children...consistency all the way around.