mentos rocket cap

assembly -
glue the two parts together and load with Mentos (i glued them in as well), drill a 1/2 in hole centered in the bottle cap and spot glue the tray in place.

launch -
drink or dump a small amount of the diet coke so that the cap can be placed without the mentos touching the liquid.
screw cap on then flip the bottle turning the top away from you or any bystander and drop/ press forcefully down on the ground while releasing your grip.

edit: so tried this with my kids, and it kinda worked. it stayed sealed but I had to hit it hard enough to shatter the cap for it to launch.