Once at a small airfield in Florida, while doing circuits at night in a PA28 at a deserted airfield, I stopped on the taxiway, shut down the engine but left all the lights on, applied the parking brake, hopped out, and did a #1 onto the grass.
(didn't stray off the tarmac for fear of scorpions / snakes / crocodiles and other nasties)
Then there are empty water bottles one uses in the cruise...