It has no physical contact and makes exercise seem like a present. This can be solitary or you can always put a guest in your canoe and hand them a paddle. These are upper body exercise sports. There is only a moderate chance of injury, all of which is over shadowed by the fun you are having. To get a work out you will need to be out on the water most of the day as this is not a very aerobic sport.
Tennis can be solitary or played in pairs.
I know, you'll say it takes 2 to play, but most of the time, if you play singles, you are thinking about how to beat the person on the other side of the net. If you play doubles, you will need to play for a longer time than singles, which should take 1 hour to get a physical workout. Tennis is a full body exercise and is a thinking man's or woman's sport that boasts more fun per stroke than you can imagine. Take a series of lessons and go for it. A great sport that can be played indoors, then moved to an outdoor court for the summer months.