Only the coin and supplies instants can be used directly from the inventory. The the time boosters can be used by clicking anywhere you see a green timer. If you click on a building under construction, the time remaining will be shown as a green button. Same with scouting a province, troops training in the barracks, the timer on a tournament and any factory productions.
Your best value is likely going to be any building that takes a long time to upgrade where you might benefit from having the building done faster, like the main hall, the barracks/training grounds, guest race portal upgrade, factory upgrades etc.