hmmm we have just come back from a week in the lakes, staying in pooley bride by ullswater. my tiddler was just over 11 months at the time, but an active walker. the weather was poor and i have to say we struggled.
we took the ullswater steamer, but the one we took outbound wasn't great as it had open railings to the side so she had to stay in the buggy all the way (im a bit of a worrier!), but the one back was totally enclosed so she could run about and have fun.
we walked up to aira force (husbands idea rather than mine - i'm 5 months preg), with her in a backpack, which she hated. managed to find a spot near the top where she could stretch her legs, but this wasn't a trip she enjoyed. and beware the tea shop and parking. both massively over-priced! we did try walking round ullswater with the buggy, but ours isn't an all terrain buggy and we had to turn back pretty early doors.
one day we drove down to ambleside and found a lovely park where she could run about, but this was on one of the few days where the weather was ok.
other than that, we called in at the tourist info in penrith and picked up lots of ideas on things we could do with her, that possibly could be done anywhere. there are quite a few petting farms, but we didn't try any of these as we do this quite a lot at home in kent. also there is a children's soft play area in penrith, but obv this is a 20 mile-ish trip from windermere. i'm sure things like this are all over anyway. i'd really recommend the tourist info folk though, i'm sure they're as helpful all over as in penrith and the lady there couldn't do enough for us.
sorry, don't want to sound too negative as the lakes are fab and we'd defo go back, but when the weather is poor it is difficult to find nice things to do with an active toddler. have to concur with a previous poster though, she had an ace time just spending proper quality time with her pa. not something she really gets a chance to do too much off in the normal run of things.....