Many merit badges take more than one weekend. It is why gaining advanced rank is such a big deal.

Lots of doing is better than lecture or power point.

Some fun stuff for lesson plans.

Teaching cotton vs wool for wet weather insulation. Have kids stand in tray of shaved ice with a bit of water while wearing a wool sock and a cotton sock.

Fire starting with real flint and steel (not mischmetal). Take a worn out file and strike it again a large chunk of white quartzite, or chunk of chirt, or flint, into steel wool.

settling water pre-purification. Find some pickling alum and smash it to powder. Have two small glasses with equal amount water. Add a big spoon of dirt and stir. Then stir in alum in one. See which settles first.