Walking at the mall is great when it gets really hot out (or is really cold and icy out). You can take the stroller and push her around (will she sit in the stroller still?) I watch a four year old, a 2.5 year old and have my 10 month DD one day per week. We walk to the park the bigger kids hop in the stroller. I wear my DD and push them. It's great exercise for me!
I'd also do the same thing outside when it's fairly nice outside.
Do you have a bike? If so, you might want to get one of those bike strollers for her. Does she ride a two wheeler by herself yet if you don't think she'll do the bike stroller?
Finally, I'd also spend some time in the pool. You'd be surprised by the amount of calories you can burn just splashing around the pool. You can also purposely add some exercise to your movements. Not standing in the pool - instead using your legs (or your arms) to keep you afloat. Things like that.
Good luck!