Although, if doing ltc style; I'd have them sign into the box, too for credit.

Maybe offer a bonus for extra credit? :-)
I wouldn't necessarily incorporate the fact that this is called letterboxing; I feel that to properly protect others boxes there needs to be more conversation on stealth and respect than might make sense in a biology classroom. But those who enjoy the hunt might look more into it and learn a new hobby. Those who wouldn't respect the hobby would just have fun on the hunt (or not, depending) and move on. :-)

I was planning small groups of 2-4 depending on the class size. I emailed our course coordinator to see if he'd prefer that I keep them in a large group, or let me break them up. If he prefers a larger group, I planned to put each group in charge of finding one box. Group 1 gets us to box 1. Group 2 would then get us to box 2.

Personally, I'd love to let them be small independent groups.

I was a bit worried about actually talking about letterboxing. I may refer to it as a campus scavenger hunt using stamps to confirm they found it.

If any students seem to really enjoy the activity, I'll tell them individually about letterboxing.