What are some tips for working out at home with an infant? Do you just put them in their playpen?

I did that for a little while but now my son is over 20lbs and growing faster than my muscles haha @beebaby,

He hates it just cries every time but I really don't know what else to do. I'm really out of shape and I feel like I'm starting to get looks from my family for not doing anything about it. @beebaby,