If you use instagram check out playfulheartparenting! She has some great silly games you can play and interact with the 3.5 yo while still taking care of baby
Coloring pages and Stickers! I also gave my daughter a leapstart which distracts her for like 20 minutes at a time
Wear your baby so you can still be up and able to play/supervise. Try a kids exercise video as an alternative to cartoons. Get a CD or have a playlist (same songs in same order) of kids songs that you can sing (and possible dance) to together.
I have a 5 yr old and 5 month old. I found drawing is fairly easy bc you can direct and help minimally. I know it’s more exhausting but I found role playing to be helpful. Even if you act as a sick patient or something
When your newborn is sleeping, play with your toddler, like cars, puzzles, coloring, etc…