Best pet besides cat or dog? ?

i want a pet that goes in a cage, so my dog doesnt get to it... im kinda over mammals because they are way too hard. especially because i am studying all day and i have to do chores all night and i have things to do over all that, so i cant have a pet that needs alot of attention. i was thinking fish- but what if i have to buy the tank, filter, heater, PLUS the fish? i was also thinking some kind of reptile or amphibian, for ex. - a lizard or frog... any advice or thoughts on what i should get?