If you want to this for the fun of experimenting, than just try something that you'd like to eat and see how it works. :)

Watermelon seeds used to be good play with, but the grocers only sell the seedless ones now. Although my BIL came up with a seeded melon last weed! Last year I got some canteloupe seeds to sprout, but the ra(t)bbits ate them.

Seeds need to be left on the plant long enough to ripen, the "seeds" you find in cucumbers arent mature enough to grow. You can occassionally find heirloom tomatoes at the organic stores, all you can do is try!

You can get basic veggie seeds during the spring for less than a buck, sometimes even 10 cents!