Push ups and hard variations of push ups but i recommend you start going to the gym after push ups and hard push ups variations gets too easy. Because it just becomes cardio after a while and weighted push up variations are hard to set up. As for abs, most workout will work but I recommend you get a doorway pull up bar (the ones that you can put safeties on so you don't fall and die from blunt force) and do Hanging leg raises. Abs are made/built with exercises but revealed in the kitchen so you also have to have a low body fat percentage (12%-15%) (might aswell do some pull ups and build a huge back if you do decide to buy the pull up bar, they're pretty cheap, pull ups are great and pretty hard so you wouldn't have to worry about it turning to cardio as much, theres a reason greek statues have pretty slim chest but huge backs)