Anyone who was a kid during the late 1990's might remember little electronic pets that was all the rage. Owners would eat, play, pickup after, and all around just take care of their pets to keep them alive as long as possible. While the fad is gone now a days, the idea is still alive in smartphones and is good time waster.
The "game" is simple, keep the tree watered and fertilized to keep it healthy and alive. Rainstorms are the only hazard is the "game" which can over-saturate the tree and cause its death. Else the players just needs to water the tree, fertilizer when the trees health lowers, and pick the fruit the tree produces to pay for the fertilizer every once and a while.
Its a simple idea that needs little over watch to play and there are only a few stages the tree grow into and no timer to keep check of how long the tree has been alive. It brings some nostalgic fun but can be boring for some but since it's free there isn't much to complain about.