When I was a SAHM, I lived for the free 'kids' magazines you see at the library or at the front at Giant or other grocery stores. Baltimore's Child, Maryland Family, Kidstreet, etc. These are full of stuff to do for the month. They list all the various activities of the local kids places. There is also tons of stuff offered at the library that would work for both your kids. Check the malls, I remember that Marley Station used to have a kids performer there once a week (free) and I think Arundel Mills and Columbia Mall does sometimes as well. Go on line and check free kids activities Odenton, MD and see what comes up. We're getting into fall so there will be tons of farm events. There is still plenty of time to take the kiddies to a pick your own farm and get some apples.
I was forever on the lookout for cheap/free things to do with my boys and with a little leg work to find some sources, I found loads of things.
You may also want to look into a moms group. There are a bunch in your area. I think MOPS (Mothers of Preschoolers) has a local 'branch'. Making friends with moms the same age is great plus there a great source of new things to do.
Good luck
Julie