Juggling.
Seriously, when I went from 3 to 5 balls I developed noticeable shoulder and arm muscles suddenly as I was doing a lot of juggling. You get up a real sweat by doing it. It feels like it gives you muscles in a functional way, rather than in the brute force weight training way, which usually seems to aim to get particular silly muscles big so you look like Arnie.
Assuming you’re not already pretty good, you might not be able to do 5, as that is jolly hard sitting down, but learning to do 3 sitting down, and then 3 lying down on your back is a good challenge.
You need a large number of medium sized balls or beanbags. If you’re not very mobile, I’d recommend beanbags that are shaped so they don’t roll too far (eg.pyramids), as you’re going to be crawling around trying to catch them. If you get good at it, you can use heavier things, which gives you more arm strength, you can get massive beanbags that are something like 750g – juggling with those for any length of time is like a serious weight session.
Oddballs can sort you out with toys (either on the net or in Camden / Brighton):
http://www.oddballs.co.uk