Play in the dirt: Bury lengths of PVC pipe (these are for truck tunnels.
Available for free or cheap at all home improvement stores, PVC pipe is totally great for home storage and organization in different ways. Take a look at these DIY PVC pipe storage ideas for inspiration.
Solar Lights for the Patio. 3 inch pvc pipe (cut in differnt lenghts). Liquid nailed them to a patio block. Painted them and hot glued the lights on top. Very easy and inexpensive.
Got tired of losing my drills in the shop. 3 inch PVC pipe solves that problem.
PVC Drill/Tool Holder using 3 inch PVC pipe
I love this idea for making mini truck tunnels to create a little construction site in your backyard. (Link goes to list of other outdoor fun ideas for kids--outdoor kitchen)