When studying about the Metric System, I do a whole day for a fun math lab. But for Customary it's a bit different since the students are a lot more familiar with it and we need to reinforce it just a bit rather than go so in-depth with it. So after our notes about how to convert within the Customary system, I give my students a challenge:
Using NO scissors, glue, tape or staples and ONLY the ONE piece of construction paper, they have to create the most aerodynamic paper airplane that they can. They have to write their name on their plane as well. I give them FIVE MINUTES.
I write their names on the board along with their estimates and then we head out to let 'em fly!
This activity is a fun break if you have a long class period like mine, but can also be expanded to encompass more Customary system activities.