Most of my middle school students referred to any conifer as a 'pine.' So I asked them to gather samples of evergreens (a nice activity to do before the winter holiday!) and we a good collection of pines as well as spruces, firs, hemlocks, cedars, yews, junipers, and larches (We're in the Northeast US). Rather than give them the field guides, I had a graphic organizer for them to use as they compared and contrasted the samples, specifying the length of the needles, whether the needles were single or in clusters (and how many), the appearance of leaf-like structures if no needles), the color of the needles (dark, light green, other markings), color of the bark if included, a sketch, etc. At that point they used field guides to identify. To take it a step further, the students then had to look back at their notes and find patterns that would help them identify these trees. For example, most pines have needles in packets (both start with P), spruces have single sharp needles (S in both words), etc. We tested their ideas with additional samples. And... the lab smelled wonderful!
I got this idea from a birding workshop at Cornell, in which the instructor (who was from Europe) noticed that American birders would immediately grab a field guide instead of observing and noting the characteristics of the birds. On the hikes, we had to verbalize the markings we saw, the sounds, size, behaviors, etc. It make for a more intense experience in observation and analysis instead of having our noses in a field guide!