To tell you the truth, I didn't find the dump that impressive that day. The piles weren't very big (nor very smelly):
Although this pile of corrugated cardboard was pretty awesome, such that one kid asked if they could climb it:
The thing that really grabbed my kids' attention was the discussion of hazardous waste. The haz-mat man in his white protective kevlar suit talked about how rechargeable batteries can react when they come together, and can ignite or explode. Likewise, old chemicals hanging out in your old grandpa's basement for years can separate and become so dangerous as to warrant calling the bomb squad for safe handling (which happens several times a month)! My kids' ears perked up for that part, and when we talked about it later this was their main take-away.
One family had smartly brought a basket of batteries with them (why, oh, why do I never remember to do this?), which led to a discussion of why we need to cut back on the throwaway stuff in our lives. It would have been a perfect time for Jack Johnson to step out from behind one of the garbage piles and get us all singing along to the 'Three Rs,' but alas, he must have been booked elsewhere that day. I'd imagine most of the good moms and dads who took their kids on the optional field trip had the song all cued up in the car, for the drive back to school, or else they were prepared to launch into EnviroMom Speech A: The importance of Reduce/Reuse/Recycle.