A few:
1. Second Poundland. LOADS of great stuff to be had for cheap.
2. Also agree with tents and forts. If you don't have a proper tent, blankets will do nicely.
3. A pack of cards and maybe a cribbage board. Children don't have to be very old to understand simple card games. My mum used to have a book titled 365 TV-Free Activities to Do With Your Child. One of them was inventing a card game. The book said not to be surprised if your child plays a two-suit stomp on your last discard, picks up all the black cards, and says "Bingo" 
Some card games, such as cribbage, cheat, poker, war, and hearts, can also introduce children to concepts of strategy, counting, and probability. (If you don't like the idea of gambling, you don't have to use actual poker chips or real money. My brothers used to use the tabs that came off cans of fizzy drinks.)