DIY Mini Foosball Table...using clothespins and ping pong ball!
Toddler Activities That Require No Setup
Calming bottles - these sensory bottles are great for little ones to explore and for preschoolers to use during a cooling off period.
Bug Hotel, something the kids can make.
A fort using a hula hoop.
Making a mini pond http://sunnydaytodaymama.blogspot.co.uk/2011/08/making-mini-pond-in-our-garden.html
A Million Cake Pop ideas- celebrating 5 years of cake pops with Bakerella!
50 Things to do in the garden this summer
How to keep a child busy for hours in the summer: DIY: Giant Ice Cube Awesomeness - filled with plastic animals, dinos, and such. Make it a learning experience by talking about the ice age first :-)