One of the most effective digital ways to generate ideas is to use generic random word generators found online. However, the Super Mario Maker community has refined this concept.
Build an entire level around the "lava-skating" and "play dead" mechanics of the Shell.
Sometimes a level sucks because it lacks purpose. Give your level a story, and the mechanics will write themselves.
Any generator must avoid infringing Nintendo’s IP; using plain descriptive text (no sprites, no game code) is safe.