You can exercise your brain, which would logically increase your ability to imagine. One way to do it is to play video games, another might be fencing or other sports. But, sports are also very physical. Video games focus on the mental reflex.
The only physical part is in pushing a button or moving a joystick. I've successfully used this as a warm up for important tests. Taking a test in school or on the job for a promotion is always very stressful. The key to testing well is to react fast to questions and not mull over them or procrastinate. You have to go with your first instinct and it has to be right. To do that you need mental acuity which can be developed with mental exercise like playing video games. Don't study the night before a big test. Don't rest all day. Play video games. Though, being well rested is important, too.
Will this help your imagination? Maybe not. Imagination is more the right brain creative side. But, still, mental exercise is mental exercise. I would think reacting in video games requires some creativity in choosing the right path to take. There are probably some games that are better suited for creative exercise.
Then there's always doing other things creative, like painting or building or even fixing a car.