Are the genetics of sunflowers mendelian? What traits are easy to observe and have known inheritance?

Sunflower traits are Mendelian, but major complications exist for making it a school exercise. Most commercial sunflower seeds,  are hybrids. They are made by crossing a male-sterile line (maintained with a restorer line) and a female line. The resulting F1 seeds are then sold to farmers for growing.
If you purchase seeds in shell for eating that are still viable they are likely to be the F2 progeny of the F1 plants grown by farmers. They should segregate with many traits in mendelian ratios. However since many genes in sunflower are multi copy due to ancient polyploidy events  expect some more complex ratios than 3:1 for some traits. Depending on the source of your seeds this is not 100% certain however that you are getting F1 seeds. The seeds could be mixed together from related varieties so the seeds from the same package might not all be F2 siblings. How many traits can be easily seen to be different by students is going to be another unknown.
If you buy seeds in a packet for home garden planting, it is a bit less predictable what you would be getting. You could be getting F1 seeds that would not segregate as they are all genetically identical. Other seeds, especially “heirloom” seeds that you might be able to purchase might be actual populations of related individuals, and different traits would segregate in different ratios depending on the gene frequencies in the source populations.
So in summary it really depends on your seed source.