Hmm...the new things that she's doing aren't really glaringly new. Except that she can sit up now. She's not very graceful about it -- and get her excited about it, and she'll kick her legs (and tip herself over backward).
The other new things are intangible this time: If you snuggle a toy into her arms, she looks both at the toy and at you. If you drift a blanket over her head, she'll laugh, shake her head, and smile at you. If you say "Come here" in a "come hither" kind of voice, she'll laugh and smile -- all of these things seem to say to me that she understands that not only are you the person who is doing these things to her, you're doing them with her, to play with her, to make her happy, and she likes that. It isn't that she's just playing anymore (although sometimes she just plays), it's that she's playing with someone.
This, too, feels good.