Some of this will tag onto Nervy Girl's post... but I have some indoor (or 'out on the deck/driveway/sidewalk' ideas) ideas. I agree- it's hard to get out and about with a little one at home.
I bought a large, rectangular flannel-backed table cloth and put it down on our hardwood floors and turned my kids loose with play-doh. The rule was that it had to stay on the "mat". Clean up was much easier and the kids were able to get really creative!
Cups, cups, cups! Buy a bag of 5 ounce plastic cups, the disposable kind, but you can re-use them, (wal-mart, dollar store) and have her make pyramids of stacked cups. Then knock them all down! Obviously, not a good activity for when baby is napping, but you get the idea.
Get a shoe-box size tub with a lid (or bigger if you want) and fill it with dry beans. *This activity requires supervision with a 3-year old*, but have her scoop the beans into a dish, a cup, use a ladle- etc. Or, just let her run her hands through the beans for a great sensory activity.
Put a step-stool by the sink and let her wash her tea-set.
My daughter used be in charge of diapers. She would take them out of the package and stack them on the changing table. I would also let her fold the baby blankets. She loved helping this way because it made her feel like she was a part of her brother's care.
Finally, you can get her started on something and back off for a bit. For example, tell her to "make a snowman out of the playdoh and let me know when you are done. I'll be right over here doing -fill in the blank-."
And I totally agree with Nervy Girl- take care of yourself, too! Don't feel guilty telling your daughter that she needs to play by herself! I think if you jump start some creativity for her, she won't be so tired!