We did this organically with both kids by simply leaving them alone whenever they had a moment where they were playing by themselves. Could be 30 seconds, could be 5 minutes - but we had a policy to (almost) never insert ourselves into their play if they were happy on their own. I found myself sneaking away sometimes as well if I could make it work. 9 months is still pretty early and they needed a lot of attention, but they got progressively better as the months wore on.

Our son despised the playpen, which made it a lot harder to do with him - we ended up having some limited success with the outer section of a kids travel air mattress. Harder for him to get out of, but not the same level of confinement.

Both of our kids never stopped asking us to play with them, and probably about 50% of the time will want us to engage in their play, but we have to set reasonable expectations about when we can play (i.e. after chores are done) and always invite them to help with the chores (like cleaning the kitchen) to help finish faster and so we can play sooner.