When Julia started 1st grade, Liliana wasn't even in preschool yet, but definitely ready for it! So, I made up what we called "mommy school." We still affectionately call it that. We would pick our "letter of the week" (as they do in preschool) and plan some activities around it (coloring sheets printed from the Internet, practicing writing it), "scavenger hunts" (well, just looking around for them, anyway) for things that start with the letter (can do this at home, or outside...say, on the way to a park or something).
Also, I like to find things in the community to go to. (Such as, story time at the library.)
Another favorite for us is making our own play dough. The fun of this is trying several different recipes. We have a couple of favorite ones.
Also, cooking together is fun (a children's cookbook can be fun to look through and pick ones you want to try).
I think it's fun to mix in some of these activities with "school-like" ones.
I hope this helps!