I've always fancied owning an aquarium, but I know my cat would eventually find a way to kill the fish... So a few months ago I downloaded the Fish Farm app for my iPhone.
You buy, sell and breed fish to gain money to buy more exotic fish, there are fresh water, salt water and big fish tanks, loads of fish species, tank decorations and 150 levels (currently on level 142).
Seems to have kept me entertained on bus and train journeys to and from the river! And it's free, more information and download here.