Congratulations on your new puppy! For mine I got him a metal ex-pen at Petsmart that is octagon in shape and allows him enough room to run around, have his food and water as well as a puppy pad. Everyone is different but that seems to work great for Oliver! It even attaches to his cage and folds up sooo easily. I sometimes use it as a gate or to keep him from going down the stairs
As for harnesses, EVERY single puppy harness (even tiny ones) were too big for him when he was 12 weeks (and is still a bit big at 14 weeks). I actually went to Petsmart and got a ferret harness and it works amazingly well! It fits him like a glove and they come in lots of different colors which was pretty neat
I'm going to get a nicer harness for him when he's full grown but for now this harness works great! Good luck!
Also, Oliver's first toys were plush squeaky toys and he LOOOOVES them!! Or any type of tiny rope too because he was teething and liked to nibble on that, which was much better than nibbling on mommy's fingers! Any toy should work great though, Oliver also loves his mini tennis ball