You're singing my song, but you are in a tough position because you will come off as criticizing your daughter-in-law if you come on too strong.
Most teachers will tell you that kids who are over-programmed are less creative, less flexible, and sometimes over-stimulated. There is a tremendous pressure and competition among parents to ensure that we are providing every opportunity to our kids, and for some people, that is measured in terms of the number of programs in which they are enrolled. If there's not an activity every day, you're a failure as a parent.
My husband and I, as somewhat older parents (37 and 41), fought this Big Time! My husband is a writer and values his private/quiet time to think and create. I didn't go to preschool or after-school activities, and turned out to be an honor student even without all that "help." At 5, our son went to religious school one day a week, and was allowed one other structured activity. Sometimes it was soccer, sometimes it was basketball, and so on - we varied it so that he had a chance to experiment with different things. We were in a preschool play group once a week. We occasionally had organized play dates, but when he wanted us to set up cross-town dates on a regular basis, we said no. He could find a playmate in the neighborhood, or he could stay home. He became incredibly self-sufficient, and very creative. He designed elaborate things in our basement play area, combining various toys to form incredible play-scapes (he combined Legos, Brio trains, Hot Wheels, and recycling debris to create huge set-ups). He collected ants and built an ant farm. He dug for salamanders. He netted frogs. We hiked, went to museums, looked at clouds, watched family movies, played board games, and so on. He learned to be comfortable with himself, to amuse himself, and to challenge himself. When he got older, we subscribed to Popular Science magazine - and he occupied himself a lot.
Despite the fact that we never pushed him in a particular athletic activity, he wound up discovering the track team in high school (after checking out basketball), and he found a great coach and developed into a record-setting track star who got a scholarship to a great college. It was all about having him find his passion.
As for all your grandson's activities, I think swimming is critical for safety's sake, and if he turns out to love it, that's great. Karate isn't bad from the standpoint of discipline, and if he loves it, that's great. Tennis isn't bad either, but it's tough when one is so young.
If your daughter-in-law is working outside the home, then daycare is essential. People can't live on one income anymore. She may be trying to give him some variety outside of the daycare program by signing him up for some different programs. She may be running from home to job herself, and feel like she's on a treadmill, so she is putting him on one too so she can ensure that he has variety, and she may feel she has no "down time" herself, and needs him to be involved in something so she can get errands and chores done without neglecting him. When she gets home from work, she probably has to make dinner, start the laundry, and think about starting his homework when he gets to school. It can't be easy for her. She may want to put him in something more wholesome than after-school TV and video games - and she should be commended for that. Economically, she may not feel she has any options.
If you are nearby and can help out by providing an after-school outlet for him, including some free play and down time, that's great. If you can give him an afternoon at the library to choose a book, hang out and read together, that's great. If you can give them a museum membership so they can visit whenever time allows (and not feel that they have to stay all day to justify the admission charge for a one-time visit), that would be great.
Try to offer her options without sounding like you are criticizing her decisions and choices. It's a different world for most parents - there are pressures to be resisted, but there are economic realities that make child-rearing these days a whole different ballgame than in the past.
Good luck!