YY to waterproofs and park, also Library
For the park, take a container to collect conkers, leaves, catkins etc according to the season
Have you got any museums locally? DS has always loved them, the smaller and more esoteric the better.
Not quite free, but very cheap, DD just loves going out on the bus, to anywhere. Sometimes we get a Day Rover ticket and get on the next bus going anywhere, then the next, maybe a train etc and see where it takes us. Not for potty training children though!
We went to a few parent/baby/toddler groups. Some were free, non cost more than £1.
DD enjoyed pairing socks as a toddler. She also had a lot of fun with a bag of pegs and a pile of tea towels.
Large sheet of paper - old wall paper is great -. Get child to lie on it, draw round, colour in, decorate etc to make paper children.
Baking
"Foreign" days - chooseba Country to learn about. Make a flag, dress up, learn some phrases, cook some food. This may involve a trip to the library or Asian supermarket.