G.W.
I would suggest checking with your local library. The Chester County library has weekly classes that are very eductional and free. I've seen computer classes, story book times, art and crafts and even sing alongs. A lot of the classes require that you register prior to attending, but it definetly worth looking into. You can create your own schedule each week. I've been taking my two young children (not yet in school) to the story book times geared for their age group. It's wonderful! My older son (6yrs old) has gone attended the chess club a few times.
Another idea would be to check with you peditrican to see if he/she knows of any "new sibling" classes that she could attend over the summer. It would be great for her to learn what it's going to be like to be a "big sister".
I have to say you are smart for getting started with planning early. Summer will be here before you know it.