Floor Play Time

Once your baby is able to sit up steadily on her own a good play activity for her is floor play time. You can lay a thick comforter for padding (if you have carpet that should probably be cushy enough, or you can lay a soft blanket on the carpet as well), then put a few of your baby's favorite toys around her and let her have a blast!
Most babies enjoy sitting, especially when they first start to be able to do so by themselves and they also love having a bunch of toys around them they can play with and explore at their leisure. This play time will also give you a break so you can do your own playing like check email, read a book, relax, watch TV, or if you are a kid at heart, you can get right on the floor with baby and play along!
Of course, once baby starts crawling you may have to adjust the play area and be ready to go fetch her if she gets into something she shouldn't. Another option is to get a 'baby corral', a play area with walls to prevent her from taking off. You can find a variety of different baby corral styles at most babies stores and also online at stores such as Amazon.