exothermic reactions are those that give out energy (most notably by heat) when they occur
endothermic reactions require energy to take place and thus feel cold to touch as they extract heat from...
The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed...
A model aeroplane is used for presentations and testing. Model planes are also used for fun. They are used model-plane clubs, aeroplane company headquarters and studios. (Bedroom, in my house)