What about Impatiens walleriana. A quick to flower annual. Available at all garden centres. It is a bit late in the season though and the plants are hybrids itself so cross polination would lead to a "pandora's box" of colours.

Other option if you have some more time you might be able to obtain open polinated pansy seeds (harder to find, but often still available as the cheapest pansy packets at retailers (several grammes in one packet and not sold in packets with 25 seeds).
Crossing pansys from two different packets gives a hybrid that could have a different flower colour than any of the two parents.

radishes are many different types and crossbreed able .
let us know how you make out .

I know this is months late, but my advice is do something that gives quick results and is repeatable -

Maybe beans -

beans have a huge variety of colors and designs.

example calypso beans that come in black/white, and red/white as well as many other type of beautiful beans.

he could also find beans that have colorful pods and see how that turns out

the plus to it is that they would grow out quickly so he could make this a year long project.
Danny

What are you,looking for ? Most interesting? Most extream ? Most colorful ? How much time do you have ? pansies are nice as mentioned . Also all corn species are cross breadable and can outcross to wild types . these inlude : asain mini corn (most ears per stalk up to 40), strawberrry popcorn ( most rows of kernels up to 38 rows)
Giant kernal corn such as Peru giant This one takes a full six months to mature , sky scraper "tallest stalk "type . there are longest ear types also sweet corn and others.

Regarding the giant corn of Peru:

Staircase Farms of the Ancients. National Geographic (1916)
Cuzco Corn
O. F. Cook

The usual behavior of the Cuzco corn in the United States is to produce plants of enormous size that mature very little seed, often none at all. It has been taken for granted that the size of the plants should be in proportion to the enormous kernels, and that our seasons were not long enough to permit this type of corn to mature.

But in Peru one does not see these gigantic, infertile plants, nor any indication that the corn crop requires a large amount of heat to bring it to maturity. The impression one gets from the Peruvian corn-fields is that the plants are not taller than with us and rather more slender, the most striking peculiarity being the prevailing red color of the foliage. The best development and largest ears of the Cuzco corn are found in some of the higher valleys, at elevations between 9,000 and 11,000 feet, in districts where the summer climate is cooler than in any of the corn-growing regions of the United States.

Thus it becomes apparent that the possibility of utilizing the Cuzco type of corn in the United States is still practically untried, because of our lack of information regarding the normal behavior of the plant and the natural conditions to which it is adapted. As might have been expected, if these facts had been known, the best results thus far obtained from the Cuzco corn in the United States have been in California, in the cool climate of the coast districts, where there is too little heat for our eastern varieties to thrive."

Karl

Here is a link that might be useful: Cuzco Corn