My kids LOVED my yoga ball. It was great for leaning them backward on it and gently rolling them, while holding them steady on it. I also had them balance on it (sitting with me helping to avoid falls) and holding their hips while bouncing them on it. (<--Hold the yoga ball in between your legs, while sitting, so it doesn't roll.) We also rolled it to one another, which taught them to take turns and share.
There are also exercises I found online that were for me and them. My kids love to have indoor picnics- just lay a blanket on the living room floor! :) They also love to have parades in the house with everyone taking turns to be the leader, and taking turns with different musical instruments.
Dance! Oh, how they love dancing to fun and energetic music. Plus, it's good exercise for all.
You could build a clubhouse with a blanket (like we did when we were kids) and have him crawl or walk under. Put him in the tub with some paper and paints for some artistic fun before bath-time. Teach him about different textures (inside or out) by having him feel different things, like a blanket (soft) and a rock (hard), carpet (bumpy) and floor (smooth), etc. Go over different animals and their sounds. Maybe make a photo book of some animal photos from the zoo. Perhaps, this could be a project: plan a day at the zoo taking photos, then another day putting together a simple picture book with the animal name under the photo. Have a photo of him at the zoo in the front. :)
Anyway, there are so many cool things to do. Let me know if you want more ideas.