library - often there are free rhyme time or story sessions, or just spending an hour there letting them run round the kids section and choosing their own books is fun.
Playground/park, feeding the ducks etc
Look on netmums for more toddler groups etc in your area. Depending on your budget there are things like musicbugs, tumble tots, art groups etc.
Painting, playdough, drawing etc are all good at this age. We rip open old cereal packets so DD can draw big pictures on the floor to her hearts content. She also got an aquadraw mat for xmas which is fantastic - no mess, they can paint and it dries up so they can use it again and again! I also print out pictures from the ceebeebies website for her to colour in.
Playdates at other mums' houses are a great way of passing an afternoon, it just sucks when it's your turn to host...
A picnic in the park - as long as it's not raining this is great fun!
Soft play centres
reading, jigsaws etc. DD is 22 months and LOVES jigsaws at the moment, just really simple ones, she will do them again and again.