Here are two challenging body-weight exercises you can do just about anywhere. These exercises will improve your strength, balance, and cardio. At a gym, you can combine them with some core strength routines for cardio-intensive circuits. Descriptions and videos are provided on the EXRX.net website.
First is the Split Jump plyometric, which will raise your heart rate quickly. This involves simply jumping in place while alternating your legs forward and back, allowing your knee to touch the ground. Start with sets of 15 – 30 seconds, or exhaustion, which ever comes first. A circuit example is Split Jumps combined with sets of overhead press and hyperextensions (or deadlifts). You can up the Split Jump intensity with a medicine ball.
Second is the Single Leg Squat. Start with your raised leg held behind you. The advanced version brings the raised leg forward. The super-advanced version adds weight. Combine Single Leg Squats with Thrusters, or Wall Shots, for a total blowout.
Create a rowing circuit by working these two exercises in with 500 meter erg intervals.