I agree with lots of pps too. My LO is almost 20 months and loves the playground. It is more of a toddler playground like Brenda mentioned, and I still stay right with her, but it is really important for her to get that activity. Sometimes when we go, DD would rather just run up and down the hills and down to the pond to throw in rocks instead of playing on the equipment. I always think we could have just run around at home, but toddlers do love to explore and it's great to get away from the house a bit. We have done morning and afternoon, but I find there isn't as much time in the afternoon, as dinner needs to be prepared, etc., and burning that energy in the morning is great.
If you're not sure about the playgroup, I would definitely try a swimming or gymnastics class. We've done both and DD loves those too. I find the structure of class nice since as mashimaro mentioned, the interactive play doesn't happen yet. Again, she and I are together the whole time, but she has that exposure to other kids and she also interacts with the teachers, which is very important as well.
I agree with you, that if he's still on two naps, it's sometimes hard to find the right time to do all these things (it was for me). But, do give it a try when you can; LOs definitely need a chance to get that physical activity in their day.