This was the bottom of the old bed (three 1x12 inch planks glued and doweled together). I added four Ikea wheels for easy mobility. Spent zero dollars on this project, only using things I already had in the house.
My youngest can spend hours with a handful of Hotwheel cars, just parking them. Seriously. I knew I had to draw a parking lot. I used three coats of black wood stain for the roadways. Then using a white paint pen I drew the lines and details to the streets.
Since one side had two way streets and a parking lot... being "the city", I decided to make the other end "the country". Brown bazzil paper glued down made for nice dirt roads. Including paper trees for the forest.
I actually had a LOT of fun building skyscrapers in the city and smaller houses in the country using legos, and our wooden block sets. The nice part is it can change every time we bring it out to play with it.
When we're done playing, we can put the mattress on top and it's his trundle bed again. Hooray for multi-purpose furniture.