I always take masses of indoor stuff for the kids to do.I'd rather go without other stuff than rainy day amusing kids stuff.My dc are 8,8 and 7 but one is a major pita when bored.I'm taking;-
loads of new books(I do a library trip before hand)
Uno
Monopoly(that can go on for hours)
Hiss
Top Trumps perhaps(always turns into a bickerfest with my 3)
a zip up bag of Lego(crucial)
new puzzle,colouring,doodle and sticker books(Usborne do tiny holiday Doodle books and this year I've got the boy/girl activity book,Usborne colour in postcards)
sodoku,wordsearch and crossword books for pita boy
plain paper,pens,pencils,paper aeroplane book etc
1 new comic each
maybe a small new toy if flush
We have a table in the tent and 3 fold up stools.I take lots of the above,as small as poss but loads so I have lots to dole out over the days that grabs their interest if the rain drags on.
Then Crocs,waterproof trousers,waterproof jackets,bin bags(never underestimate),loads of tea bags,wine,cake,choc and hot choc. Loads of batteries for lights,the dc all have those clip on book lights.
I too love the sound of pattering rain on canvas if the kids are amused and I have a good book.Crickets(or a 4 star hotel) would be better but you know it's nice being cosy and on an adventure now and again.