I've been field-testing an indoor ID activity that involves students matching up photos of leaves with illustrations of the same leaf. I have a set of 24 laminated cards that each student receives. The student has to find the other person with the same leaf as them, which requires them to mingle a bit and compare/contrast leaf shape and pattern. It's not a difficult excercise, but a great one to engage them in how to then use a field guide because they come up with the features they used to decide if something was the same or different. All ages could do it and they can then dive into the field guides to put a name to what they matched. As for the field guides, make sure you have ones that also have good illustrations or photos of the leaf itself otherwise they will misidentify or get frustrated. Lone Pine guides don't always have the best identifying features (ie you pretty much have to know what it is to look it up), Trees in Canada is excellent, as is the good old Golden Field Guides (cheap and small).