A total body core exercise

How to keep strength without any equipment? There is a complete exercise you can do with only your bodyweight. You’ll work strength in the upper body along with your entire core: the one arm push up.

This exercise need a good inter muscular coordination because your total body has to be in perfect alignment: it means you don’t use only your arm but all the core muscles to maintain the body under full tension; by this way, the movement is more effective and you develop more strength.
This exercise is not easy but you can do it elevated: spread out your legs for balance, grip your hand on the support, make a fist with the other hand in your back, contract your abs, your butt, all your body; inhale and hold while doing your rep and exhale at the end; keep your body tensed all time. All the muscles work in synergy and this exercise is far more effective than doing your pecs on a machine.
My advice, go for it without hesitation!

To learn everything and become an expert, take a look at the book of Pavel Tsatsouline: The Naked Warrior.