I already know about avocados and cherries, what else is easy and fun to grow?

You can grow anything from any fuit seeds, an easy one is a lemon tree, just take aseed and place in soil, water, it should come up. In winter, make sure it's in a sunny warm spot.
Orange plant the same thing. some fruit tree seeds will grow into wild tree of the same, than You have to graft,
EDIT-If you want to have fun with someone that is growing cucumbers, take some barley seeds, and stick them in cucumber, they will grow beardlike on the cucumber..

sweet potatoe it makes a really pretty plant . good luck .