Some suggestions:
Swimming either book him into regular weekly classes or take home yourself and have a splash and play
I second and highly recommend the Library storytime great for that age group with a story/maybe a song and a craft activity at the end it will be something he looks forward to each week
Art class
Like PP have suggested Playgroup, Music, Gymbaroo local parks,
Playcentre
Once a month go an an adventure such as a train ride, or other public transport, go out for lunch, go to a movie,
Google kids activities there are plenty of good websites which you can enter you state and they tell you whats available one in particular is very good but I am not able to say on here