Hi Charlie
I think it really depends on whether you have any bone damage and what pain you have. The Myeloma UK booklet says walking and low impact exercise is good like yoga and pilates and this would include swimming but as swimming pools can be a hotbed of germs these are probably best avoided!
It really depends on what you can do. I was pretty active before my diagnosis and once I got past the treatment stage and my back pain went I was able to do more walking, played tennis and cycled. After my stem cell transplant there was no stopping me, the Manchester 10k run, more tennis, skiing, outdoor fitness classes and lots of walking. I am now starting training for a triathlon!
I checked the above out first with my consultant and she told me to build up slowly but do what I wanted as I did not have any bone damage (turned out that I did but thats another story).
During treatment, I was really down and in pain and nearly chucked out my sports gear thinking that I would never be able to play squash/tennis again. A year later I was doing the Manchester 10k! How things change but whilst you are on treatment take it easy, a bit of gentle walking and stretching, tai chi or qi gong.
Wendy