Let me fill you in with a bit of background before I get into what I want to get accomplished. First, I'm small for a 17 year old guy, 120, 5'6". I'm a coxswain for our city's rowing team, so I don't get much exercise in life besides walking up a ton of stairs at school every day, which keeps me relatively in shape, but not a lot of definition/muscle and whatnot. However, now that school's out, I have nothing else to do. I've taken up running a mile a day, trying to get time under 6:15, but I keep getting an awful stitch a quarter of the way through that keeps me around 6:45-7:00. I've been doing a lot of "rowboats" (you know, the exercise where you sit on your and go down to a flat position, and come back up etc.) and I've lost a lot of the fat on stomach and core is a lot stronger. I've been doing pushup ladders (5,10,15,10,5) twice a day, so I can see chest is a little more defined than it used to be. But since pushups are more for triceps and chest muscles than for biceps, is there anything I can do to make arms a little bigger?
I don't want to put on too much weight, since I'll still be coxing next year, but I won't grow enough to actually row if I gain too much weight, so 5 pounds over the summer is kinda what I'm hoping will be limit. However, I want to get a little bigger, in legs, arms, abs and stuff. Not huge, but a little more defined. At least really defined abs would be nice for the beach

Problem (or is it?). I am not gonna go to the gym. I feel like I'll gain too much weight at the gym (and besides, those memberships are expensive) so i want something I can do at home, preferably something with own body weight.
If anyone can give me tips that'd be great.