Build a cheap and easy bean fort.
See how they did this here.
Set up pool noodles for a game of kickball croquet.
Hula-hoops work too. Get the directions here.
Grow a sunflower house for the kids to play in.
Have a backyard drive-in party for the kids with cardboard boxes.
Get the instructions on how to make a DIY backyard movie screen here.
Make a race car track.
Use glow necklaces and glow bracelets for a few rounds of nighttime ring toss.
Break glowsticks into bottles of water for some nighttime lawn bowling action.
Reuse those wooden stakes to create the most epic game of Scrabble ever.
Tie rope between trees to create a fun obstacle course for the kids.