As a shower foodie myself I can say:

1. Orange (an absolute classic, 10/10 would recommend. The trick is in savagely attacking the orange like a beast)
2. Beer (a bit less of a gamechanger but the contrast between a cold beer and a hot shower is very nice indeed)
3. Watermelon (same principle as the orange, a bit more of a challenge but worth experiencing for sure) ...
4. Lasagna (do not attempt. May god forgive me)