My 5 year old son won the school miniature garden competition back in June. The kids had all been doing a "green project" and the miniature garden competition was the culmination of the project. The children were each given a seed tray to turn into a miniature garden.
He had....
......a pond made from a cut-up freebie CD (they do cut quite easily with good scissors),
......a patch of grass (turf),
......lobelia/busy lizzies/other small flowering plants (and weeds!)
......a wooden shed made from a cardboard box with lolly sticks stuck on
......a greenhouse made from an upturned clear punnet with strips of paper stuck on to seperate the 'panes of glass'
and best of all, a miniature tomato plant "tree" in the corner, which is now bearing fruit despite STILL being in the seed tray with all the other plants!
Hope this has given you some ideas.
Good luck"!