Mercy, there are SOOO many challenges! These can be entered at any time:
Support the Troops - Walk to Afganistan (any kind of exercise, not just walking)
1st Day of Spring - Personal goals, no rules - you do your own thing whether diet or exercise, very supportive.
As the other poster said, look through the threads - there are seasonal and holiday challenges, weigh-ins galore, individual plans and sub-groups with their own challenges. There is even a New Year's Reading Challenge

- lots of titles for bookworms or for on-going motivation!!
Let us know what you pick!