PS Tricep dips are great, but I found much easier to use the edge of the bath, sit on edge,hands, gripping bath either side of your bum - alternate having your hands quite far apart and right next to your bottom - whilst sitting make sure knees and feet are together, and keep elbows tucked in to your sides.
Slide bottom off bath edge so you go straight down the side of bath, until you're almost on the floor (depending on if you have a weirdly high bath?!), push yourself back up using just your arms. YOu may manage 2, you may manage 5, but if you can do 20 straight off you're not using just your triceps!!
THis is really great, and does work, and I've just reminded myself to go and do some!