a kid a grow box and let them pick out the seeds of what they want to grow. Kids love it as long as their parents don't turn them off of it.

When I used to teach middle school science I would give the kids a couple of small containers and dirt and let them choose what to plant. The kids would come in 2 or 3 times a day to check on how well their plants were growing. I would tell them it had only been a couple of hours since they were last looking at them but they would still come check. There is just something about watching the whole process that is satisfying.