If the weather makes it impossible to get outside and you have a video game system, host a "video games Olympics." Choose video games that are short -- not long adventure games; otherwise, the people who aren't playing will get bored. Pick games like bowling, ski jumping and tennis.
If you plan out your video game Olympics in advance, you can set up tournaments and give out prizes or trophies to the top three finishers in each category. If you have a lot of people participating, you can even break everyone up into teams and wear team colors. Make it even more fun by setting up "fueling stations" with sports drinks, protein bars and fruit, so everyone feels like a real athlete!