I'm trying to stay physically active. Any suggestions on good activities for a myeloma patient?

Any activity (as long as your doctor approves!) is good activity. Aim for at least 30 minutes a day. Walking is terrific. So is yoga, which has numerous physical and mental benefits. Dex is a muscle wasting drug so you might be extra weak. Weight lifting is a great idea but be extremely cautious with the amount of weight so you don't injure yourself or break a bone. Definitely discuss with your doctor before getting into a public swimming pool.