Ah, joy! Our DD came home at 8 months old, nearly a year ago now. It's a smashing age.
I strongly agree with PPs who say wait and see, particularly looking out for what his life in FC has been like. For instance, DD was really pretty under-stimulated in foster care, spending most of her day in a rocker in front of the telly. Even though she's developmentally spot on, her muscle tone was pretty low as a result. She couldn't sit unaided and she was a good five or six months away from pulling herself up on anything.
Similarly, because she hadn't had much in the way of stimulation, something like a trip to the park was a BIG deal for her. She found it really exciting (in a good way!) and really didn't need any bells and whistles for entertainment.
We found a big playmat for the living room floor really useful, and spent a lot of time on it doing eye to eye contact while playing and reading.
She didn't have a playpen or anything as she didn't really tolerate that kind of separation, so in the end I got a hip seat and got used to doing things one handed!
Agree that books are terrific.
Swimming nappies would be good!
Will let you know if I think if anything else. Feel free to PM me.