As mentioned below, when I was pregnant with C, I started making a list of activities that I could do with M so I wouldn't have to wrack my brain every time I needed a fresh activity or something to get up out of the house. I'd been working on the list and my plans for the summer when I ran across this post on Zen Habits and found a lot of activities I hadn't thought to add.
A few caveats: I've left activities on the list that I have found difficult to do now that I understand the reality of having a baby and a preschooler like going swimming. We regularly do lots of crafts, so I didn't include activities like painting or using markers.
Here's my list in no particular order.
Do a cooking project
Go to the park
Make a playdate with friends
Play in the yard
Watch a movie
Do errands we can walk to (for us that includes going to the post office, renting a movie, buying bread at a local bakery, and going to the library, buying coffee beans from our favorite roastery)
Check out books from the library
Go to a coffee shop for a snack
Go to our local nature center/zoo
Play with sidewalk chalk
Pull out a toy we haven't played with before or a new toy from my stash (I keep a few inexpensive toys in a closet for special days when we need something new)
Go to the children's museum
Play with playdough
Paint with water
Do some pages in an activity book
Go to Barnes and Noble and read and play with the Thomas set.
Go see the animals at Pet Smart
Set up the tent to play in
Go for ice cream
Do a treasure hunt
Play games on the computer (like those at PBS Kids)
Dance to fun music
Water play outside
Find an educational show like on Animal Planet, Discovery, etc
Take a collecting walk and search for special rocks, leaves, pinecones, etc.