Act 1

Pound of Quarters or Pound of Dimes? from Nathan Kraft on Vimeo.

Act 2
You might want to use the US Mint site. You could also just measure out a pound of quarters and pound of dimes and then find the value. You'll need an accurate scale. You could use a triple beam balance to mess with them (it will be in grams). I tried this, but of course, I didn't have a pound of each. The results did not exactly match the actual (using the US Mint site) and I think an interesting discussion can be had from that.
Additional info: 1 pound = 453.59 grams

Act 3
I don't think you need anything from me. I think the best way to demonstrate this is to actually weigh each and then count them.