I am the type of person that always needs a new activity to keep me motivated. Recently I tried a new class our gym was offering called BOSU Challenge. This class was a blast, I literally left smiling..... but was dripping in sweat! The BOSU meaning "both sides utilized" can actually be used on both sides, either the dome or platform. Its a great workout that engages all muscles (especially your abs) as you try to maintain balance and coordination.
Check to see if your local gym offers these classes or pick up a DVD and a BOSU to get started!