Is it okay to pee in a diaper for fun?

It depends on who's doing the peeing. If it's a child, well, they're gonna pee whenever they feel like it, so just lettem do what they gotta do. If it's an adult, well, you can if you must, as long as you use the diaper sensibly, i.e. don't put on the baby afterwards :), but i would maybe question your mental health