My little boy is 3 and a half years old, just like you! Here are some activities he has been getting up to which you may like to try;
Exercise circuits around the garden:
Why not set up an exercise circuit around the garden, we included jumps, squats, star jumps and jogging on the spot, we had lots of fun!
We found some leaves around the garden and stuck them to paper. We then painted them and added glitter, my son did most of this independently which was great to see and sat adding paint for ages!
We've been using the stickle bricks we have to build a zoo for my son's animals which he loves to play with.
We've also been doing lots of puzzles and he is getting really good at them!