The most secretive body parts are muscles, which hide behind layers of skin and appear only occasionally to help lift cars off trapped kittens. Flex your biceps in the absence of a cat emergency with today's Groupon: for $20, you get a one-month unlimited membership to Mecca Fitness (an $87 value).

Mecca Fitness's 24-hour gym encompasses more than 16,000 square feet of workout areas and exercise equipment with no time limits or sign-up boards. Watch flat-screen TVs while running on treadmills, or shake up muscles and jelly-bean maracas on vibrating Power Plate machines. Exercisers select from nine different resistance levels to tone limbs on an array of muscle-building equipment designed by Strive and Mecca's resident fitness guru Grant Roberts. Meanwhile, bodies sculpt six-pack midsections at the abdominal area, women lift weights in a ladies-only workout space, tanning machines bronze bods, and athletes catapult orbs—like children throwing balls of fire at the sun—toward backboards at the basketball court. Gym members can also stock up on workout fuel at the smoothie and supplement bar.