The simplest of activities can sometimes provide the most joy for children. My 10 month old loves supervised splashing with a large tub of water on the deck. Pouring, sprinkling, floating, splashing and mixing.
For older children I set out a few containers (empty clear juice bottles are good) of water with drops of food colouring in each, and watch them mix the colours together to create new ones.
Small plastic sea animals or boats are also fun to scoop and play with in tubs.