Good idea about the cheap shower curtains!
Farm and soft play are always good contenders, but at that age I had to go everywhere with Imogen at soft play and it was exhausing!
Specific missions like "how many different types of leaves can I collect" then siticking them in a scrap book, or saving cardboard to make a robot/fishtank/scenery (tree bodies from painted loo-roll middles, cotton wool clouds etc) - it's amazing what comes back from nursery made out of cut-out handprints too! Start pinching ideas - he won't mind repeating something he did before.
get some beanbags from ELC and have him running from one to another - fetch the red one, now put the yellow one by the washing line... wear him out!
Cooking / baking always good for a rainy day. My boss reckons making of bread is great because you do a bit, which is about the duration of their attention span, then you leave it to proove, then you go back "ooh look it's got bigger" then you make the shapes (attention span again) then it gets baked then you eat it. I've not tried it yet but it's one to consider.