The National Trust has launched a lovely campaign aimed at getting children outdoors and connecting with nature. Its called 50 things to do before you are 11 & 3/4 and is full of lovely (mostly free) ideas such as eat an apple straight from the tree, hunt for bugs, skim a stone. There are activities suitable for all ages a couple of the more adventurous ideas are canoe down a river and a high tech nature hunt using GPS. I confess I have only done 47 myself but we have signed up for the the challenge and my young children have plenty of time to complete it. Sign up for yourself here.
The national trust also has some great ideas for entertaining kids on rainy days, of which we have had plenty lately. I printed out this activity sheet to take on holiday with us last year.