Try aquilegia, lots of varieties (color, size, double bloom, double color, etc.) that hybridize easily (without help, too). Seed germinates the same year and resulting plants flower the following summer. So you do not have to wait too long for results.
Happy hybridizing
Jens

It's easy to cross-pollinate zinnias, and they come in a wide variety of colors, flower forms, and plant habits. Small Kelley forceps are a good tool for transferring pollen, although tweezers or Twissors work fine too. It helps to work in the early morning to get a head start on the bumblebees.

MM

Violas...I put some in pots on a mulched patio last year and this year they reseeded all over the place. They were delightful ! SOOOoooo many different crosses and it is now July and they are just starting to look ratty, and only because it has been dry and I didn't keep up with watering them.

Adam

Epimediums are essentially self-sterile, and virtually 100% cross fertile, even across most of the other species. If you grow several Epis near each other, each year you will be guaranteed a new set of self-seeded inter-specific hybrids. I have 2 years worth of crops coming along, and should see the first blooms next spring.

You can get hybrids of a desired cross easily by planting a pair of the desired parents away from any other Epis and then all their seedlings will be the cross from those plants though perhaps in both directions. I have 3 sets of parents isolated this way to get a cross I want to look at.

George