And then I remembered cress. OK so it doesn't quite grow before your eyes but it does grow quickly. "Plant" it on damp kitchen towel one day, it has grown roots by the next day, and green shoots appear the day after that. Within a week, you can "harvest" your "crop".
We laid out two lots of cress seeds, in the shapes of the boys' initials. They were both excited to see whose cress would win the race to grow. Of course, side by side in the same container, the race was pretty much guaranteed to be a draw but that didn't spoil the fun.
A sunny windowsill and extra water each day meant that by the end of day 5, the cress was ready to cut!