Children can jump and slide on a large, wet sheet of thick polythene available from DIY stores. Spread it on a slope and leave a garden hose running at the top corner. Move the sheet every hour to prevent too much damage to your lawn.
A section from a big log makes a fine play horse. So does a section of large concrete water pipe.
For maximum fun as well as good drainage, build a bottomless sandpit. Make a frame of 10 x 2in/250 x 50mm planks; reinforce the joints by driving stakes on both sides of each corner. Within the frame dig a pit 2ft/610mm deep and fill with play sand. Cover with plastic or canvas when not in use to keep pets, insects and water out.
