At 3m frankly everything is stimulating! Clearly, sitting in a quiet room reading stories with you is not going to be as stimulating as being at a baby group, for example, but as everything is new and different for them they take in and notice a lot more and therefore everything is exciting, iyswim?
Of the list of things you do, I would say that the things which risk being overstimulating are baby groups and bath (for some babies, for others it is a great winddown at the end of the day). The other things sound like great ways to spend the day, and I wouldn't worry too much about OS from them.
If he is grizzly towards the end of an A session but then goes on to have a decent nap and settles for it normally then he is just tired when he has been awake for the right amount of time. If he was OS, I would expect that firstly he wouldn't just be a bit grizzly, and secondly I would expect it to be very difficult to get him down for his nap and/or to keep him asleep.
If you want to do more stimulating activities, then the best time to do them (as a sweeping generalisation) is just after a nap, when he is rested and fed, but over time you will probably get a feel for what particular time of day is better for him.