Socks
I read a really neat idea about putting socks on a dog's feet and taking it for a walk somewhere muddy but with lots of weeds. When you come home, you remove the socks and plant them in a pot being sure to keep them watered. The writeup said that it was astonishing how many plants sprouted from the seeds the dog picked up.
Now is a really good time for a walk through the woods to collect seeds. I would think that students' shoes, sock,and pants would collect lots of seeds. Students could also be on the lookout for seeds being dispersed by the wind. You could give them a fruit (apple, orange, grapes) to eat as they walk along and then talk about how they helped disperse the seeds of that fruit.
If a field trip (walk) is not possible for the students, perhaps you could go collect lots of seeds and decide how they are dispersed.