In my experience, grapes give rodents pretty bad diarrhea.
If you want to feed them, stick to nuts and seeds.
Everywhere I search it says that it's good to give them some greens alongside nuts, how come?
"Greens" normally means things like leafy vegetables. Things like fruits have a lot of sugar, which combined with the high liquid content, has meant very dirty rat, degu, and chinchilla cages.
How'd you know?
Anyways, we've made sure they won't become friendly or our friends by only giving them food every few days and making sure they're scared of us.
They don't visit when the little food hut isn't out either, as intended. They're fun to watch eat.