Buy a set of pushup bars and use the shit put of them. I do pushups every night when I am just chilling at the t.v. It is not hard to just roll onto the floor and start doing a few. I honestly don't think that you need a break from them the more the better. I usually don't do them the same day I work my chest, but it's for pain/weakness reasons.
As for pullups, if you go wider you will get into the back muscles, i.e. lats and delts. These will give you that wide shoulder v body look. Oh, and fo them with your palms facing out. If you go narrow with your grip you will your arms and shoulders a bit. Don't be pissed if you can't knock out any wide grip palms away pullups. Just keep trying and do what you can. They are a very demanding exercise.
Those two exercises will make you look ten times better and be a hell of a lot stronger.
Doing situps and crunches will make you feel good about your stomach, but remember everyone has a sixpack. It's just burried on most people. Don't get me wrong, they will help you and I do them every other day, but it's not one of those must do exercises.
You can't really gain to much mass without equipment. Pushups/pullups, will take what you have, add a little, and refine the shit out of it so it looks good, but not much more mass will be created. To get mass you have to rip you muscle by lifting heavy weights. Basically when you lift heavy weights your muscle gets tons of tiny tears in it and then your body makes more muscle to fill in the holes. Thus more mass and muscle.