Even though getting outside is more fun than being stuck inside, there are going to come times when you need to get some exercise but are confined to the indoors for some reason. That doesn't have to mean you are stuck doing a boring, unrewarding gym workout. Finding adventure can be easy and doesn't have to involve any special equipment.
One of our favorite impromptu workouts is the "Deck of Death." It provides a degree of the unknown and a reasonable toughness factor, while being conceptually simple and being doable both alone or with a friend. You simply need a normal deck of cards. Assign a different exercise to each suit. For example, hearts = burpees, diamonds = sit-ups, clubs = pullups, spades = squats. Now proceed flipping cards, performing that number of the associated exercise (7 of hearts means you do 7 burpees, then flip the next card). Jacks are 11, Queens are 12, Kings are 13, and Aces are 14. You can also assign something special to the jokers, like a short run or a minute of plank. The 7 Zillion scientists are telling us that you will end up doin 104 repetitions of each exercise.