We have two aquatic frogs that we've had for over three years and they're going strong. They came from the Grow-A-Frog website. Started out with one, then ordered a bigger tank and they sent another frog with it!
They're basically like taking care of a fish. I sprinkle food in there every other day......you can buy aquatic frog pellets at PetCo/PetSmart. There's controversy regarding using a filter causing them too much stress. I have one, but don't run it all the time. I have the tank on our kitchen counter and change 1/2 to 3/4 of the water 1-2x a month. In the wild, they are accustomed to being in still, stagnant water so it doesn't have to be crystal clear. To help keep it cleaner, I have a few snails that help clean things up.......those cost just a few bucks each.
Grow-A-Frog has a lifetime replacement policy where if the frog(s) die, they'll send out a new one. The 1st one we got as a tadpole and it was really cool to see him evolve.
I have the two of them in a 10-gallon tank which is just about right for them. They are cool to watch, but of course, it doesn't take much time for the novelty to wear off since you can't really handle them.
Inexpensive pet to have and super easy to take care of! I think if you're looking for simplicity, something like this or a fish is going to be your best bet.