Could you and your friends start up a reading club? Pick a book a month and get together once a week to discuss what you've read. Check online if your friends aren't readers.
What about a writing club? If you and your friends enjoy writing you can compose pieces and get together to share a critique, give advice, etc. You might be able to find on online one if this doesn't work for your friends.
Or what about a game club? A friend of mine (who is a stay at home mom) plays Bunco one night a week with other moms in her area. It gives them all time away from the kids and they have something to do together. You could do card games, board games, Bingo (my fire station does cheap Bingo every week), anything you can think of. Or just have Margarita night!
Scrapbooking is a great idea, especially since you have a child, but it can be an expensive and time-consuming habit. There are some digital scrapbooking sites which you could look into that might be less expensive. These are also good if you aren't especially crafty.
My husband and I have recently started learning French; we are teaching ourselves and it's a lot of fun. You can pick up tapes/dvds/cds from your library at no cost. This is also something your son can do with you, children are usually great at learning new languages.
Photography is another option. If you already have a camera the cost is minimal. Go to websites and learn about composition and lighting (or get books from your library) and then take your son to the park, or go for a walk yourself and start shooting. Most kids love photo shoots! If you have a digital camera, you can wait to develop the pics once you have the money to do so, and in the mean time you can just enjoy them on the computer. Set up a free Flickr account (or something similar) to share what you've done with family and friends. Once you have the money to print the pics, have them framed and hang them in your home or give them as gifts.
Hope I helped!