Nasturtiums and sweetpeas have large seeds that are easy to plant and easy to collect at the end of the season.

Squashes are easy to hybridize. You could cross a pumpkin and a zuchini (just for example). Squashes have interesting flowers, with a separate male and female flower. Try a cross in both directions. This year you will have normal fruit -- a pumpkin and a zuchini. Save the seeds and plant next year, who knows what it will look like? Or try a large and small pumpkin variety, or a warty squash crossed with a smooth one.

Here is a link that might be useful: Pollinating pumpkins

Here's an easy program for the Shirley Poppy. I ran across this article by accident today, and of course thought of you. Basically, these poppies have a very wide genetic pool of colour. You plant them, then select your favorites and allow them to reseed. The following year, you'll have more of what you like, and fewer of the ones you don't like.

This project would work with any self-seeding annual that has a wide variety of colours in a single package. Have fun!

Here is a link that might be useful: Breeding Your Own Shirley Poppies