Get on eBay or go to thrift stores and garage sales and try to find and buy all the toys and books you loved as a kid and the ones you always wanted and never got. Then if you run into things you never saw but would've loved as a kid, buy those too.
I love to have music and dance parties with my three year old. We will put tribal music on and dance all around or create music with stringed instruments, maracas, drums, etc. So much joy bubbles out of the both of us with these special times!
Creating art together has also offered profound experiences for us. Painting in nature is our favorite. It is so much fun watching his process with testing out different strokes and ways of painting.
Baking and cooking together also brings much joy to our hearts!