If you are looking for a great way to make a game more fun, you should try adding traps and puzzles.
Traps can be easy, you could have deactivated laser beams or sentries that become active when you step on a trigger. You could also have a trigger that teleports you to a room that you must escape from in order to keep playing.
As for the puzzles, you could leave “notes” (using a button and a pop up) to give players clues to where and item is hiding, what the passcode for a door could be, things like that.
For mini game ideas, you could have a hidden boss fight, a prison escape or a game where you push a ball through an obstacle course
If you have more ideas or ways that those ideas could work in a game, reply down below.