What kind of weights do you have? Here are some exercies that can be used to tone shoulders/back, biceps, and triceps. I’ve never tried them at home, so some of the couch modifications may not work. Trial and error 🙂 You said you only have 15 minutes, so you’ll have to super set. Instead of resting in between exercises, just switch muscle groups. For example do one set of bicep curls, then one set of arm raises. Also, be sure that the weights feel heavy. It should be hard right from the start because you’re only doing two sets of 10. It has to be challenging if you want to increase your muscle definition!
(Biceps) Bicep curls: Use at least 7.5lbs. Sit on the edge of the couch, hold your arms straight down in front of you, wrists facing forward. Curl straight up until your biceps stop the motion or until your weights nearly reach your shoulder. Alternate between curling straight foward and curling out to the sides. Be sure to do a full motion from fully straight to fullly bent arm. 2 sets of 10.
(Shoulders) Side arm raises: Use at least 7.5lbs. Hold your arms straight down to your sides (turned just barely to the front to avoid injury) lift your entire arm (straight) until your arms are parallel to the floor. Hold for one second. Slowly lower arms to starting position. 2 sets of 10.
(Shoulders) Front arm raises: Use at least 7.5lbs. Sit at the edge of the couch. Start with arms straight down in front of you, wrists facing inward. Raise arms (straight) until they are parallel the floow. Hold for one second. Slowly lower arms to starting position. 2 sets of 10.
(Upper Back/Shoulder) Reverse Row: Lay on your belly so that your shoulder is just barely over the edge. Holding a weight (at least 5lbs). Hold your arm straight down over the edge toward the floor, lift arm up to a 90 degree angle, hold for 1 sec, lower to starting position. Alternate sides. 2 sets of 10 each.
(Tricep) Tricep Kick-back: Lay on your belly so that your shoulder is just barely over the edge. Hold a minimum 5lb weight at a 90 degree angle. Raise only your forearm so that your arm is straight to your side, hold for 1 sec, lower to starting position. Alternate sides. 2 sets of 10 each.
(EVERYTHING!) Arnold Press: Just watch the video. I know he’s a big beefy guy, but this is a GREAT exercise that you have to do while sitting. I use 25lbs weight, but you could definitely start with 10 or 15. http://www.youtube.com/watch?v=6Z15_WdXmVw