I endorse this comment. I've been reasonably happy with "circuit training" as a supplement to any training program. I typically do a circuit of 6 or 7 exercises and run through the circuit 3 or 4 times for 60-90 minutes in a session. I like a mix of "pre-hab" work with stretch bands, like external shoulder rotations and rows, (swimmers need to take care of their shoulders) static strength exercises like planks, body weight exercises like chin-ups and hanging knee-raisers, and light-weight high-rep work like squats and curls with dumbells. Sometimes I'll mix in work on the stationary bike, but more often I'll do bike work as a stand-alone session. It's easy to put together enough equipment to do this at home so it's a great go-to workout when pressed for time. You can do a lot with a yoga mat, an assortment of dumbbells, Bosu ball, maybe a chin-up bar, etc. Opinions on this kind of exercise vary widely. Some folks will tell you that this kind of training is worthless, but I feel that it has been generally beneficial for me. Averaged over time I tend to get in 7 sessions per month, so twice a week but I tend to miss about once per month.
Re the comment above about pull-ups and shoulder problems: I think there is some validity to the concern. Pull-up and chin-ups seem excellent for building upper body and core strength, but extreme caution is advisable. If my form slips at all, or I feel the slightest bit of a tweak in my shoulder, I stop. No pushing to complete failure. It's not worth the risk.