Everyone’s advice here is really good. What works for me is:

I alternate between 100m freestyle (crawl) and 100m breastroke. This gives me time to catch my breath* with the recovery stroke, then go harder on the freestyle. I aim for about 30 laps of a 50m pool.

You can also take along a kick board and use it when you want to focus more on your legs. It’s a real workout since you aren’t using your arms to pull you through the water.

If you want to focus on upper body, you can use a pull buoy. Essentially you place it between your legs to keep them afloat, and it stabilises your hips, allowing you to focus on your stroke (pull).

I find mixing up normal laps with focussed laps helps break up some of the monotony, and make sure I’m working out evenly.

(*I also have ear problems which means I can’t sustain a submerged stroke for too long)

as with any form of cardio - choose what suits you.
e.g., if you’re more explosive and prefer to get it over and done with asap, try doing a front crawl/freestyle as fast as you can for as long as you can, take a minute break and repeat a few times.
alternatively, if you’re more ‘low impact’, then more conventional swimming like breaststroke at a leisurely pace for 30+ lengths.

same thing applies to running - when I was in my early 20s I predominantly did high intensity sprints for like half an hour - 10 years later and I prefer hiking for an hour or two instead.