My 5 year old impresses me with his lego creations. On a regular basis he will pull out the basket of duplo blocks or regular legos and start creating.
My ideas include making...um, a house, and a road, and um...another house. Then I sit back and watch my boys build much "grander" creations than I could have thought of. On this particular day, my kiddos decided they needed a home for their Zhu Zhu pets. We spent time creating their home and then the boys spent a long time pretend playing and sharing stories about their pets.
When we first bought legos, I searched all over Pinterest and found a few good ideas (preschooler playtime pin board here)...but then I spent more time looking than we did playing. Honestly, the boys have more fun when we just sit together and they create!!
One thing about parenting that I've been surprised by is that I've forgotten how to play - and have been glad that my kids teach me just as much as I teach them!