Beans, corn and pumpkin seeds are all large enough to be easy to handle and germinate quickly. Lettuce, radishes, marigolds, zinnias and tomatoes all germinate readily and are inexpensive. Often you can get just as fresh seed in the same size packet for much less money on the discount racks. Learning a little bit about seeds and plants will get you thinking about all sorts of experiments you can do. And how long it takes seeds to germinate can be greatly influenced by the temperature.
Plants can be easy to manipulate too.
The height of a tomato plant is greatly affected by the amount of light it receives. When growing tomato seedlings indoors, keeping the plants as close to the light source as possible will result in stronger, shorter plants. When working with a fluorescent light source, the amount of light available drastically increases as the plants get closer. A little reading before hand will save you lots of time in the long run. Plan on enough time to conduct your experiment early so you can work out any bugs and redo it if necessary.