I'm doing these challenge now but you can do also the 1 to 100 burpee challenge. Each day do one more burpee that the day before . I lost 8 pounds doing that. Example : day 1 burpee, day 2 burpee until you finish in day 100 : 100 burpees.
If you feel self-conscious about having really skinny legs, you need to do some training to bulk them up. This infographic features 19 exercises you can do to take your legs up a few sizes. A Skinny Person's Guide to Gaining Weight