Key in successfully sculpturing this leftover soup is that it is very viscous. Use your favorite broccoli soup recipe and increase the broccoli content while reducing (tremendously) the amount of added water.
On day 1 you'll thus serve a very thick broccoli soup (or call it mashed broccoli). On day 2 you can then shape the leftover broccoli pulp on a plate in the form of a broccoli (or any other object) by using simple kitchen utensils (knife, spoon, fork). If you didn't do it yet on day 1 then use a hand blender to obtain the right modeling properties.
The sculpturing will also work really well (or even better) with mashed potatoes (leftover or fresh).