Okay, I'll stop complaining that I have to limit my weekday swims to 45-50 minutes. (The interval between elementary swimmers exiting the pool and lifeguard turning out lights and locking up.) At times, various interruptions by students, swimmers, and parents will cut the time to more like 30 minutes. Some workouts I have done in limited time:
*A continuous swim. Start easy for the first 200m or so and then pick up the pace, ease off with five minutes to go for cooldown. At your speed you could get in almost 1500m. Do every few weeks and track how far you get. Or just do an easy continuous swim if you need to recover from a hard swim/bike.
*As others have said, repeats with little rest. For your pace, maybe 100m on 1:50 to 2:00 sendoff. I like to throw in an IM every 4-5, to help count and mix things up a little. (I'm awful at breaststroke, so I add 30 seconds to the sendoff time for IM.) One in a while I do the whole thing IM's just 'cause I get bored of freestyle.
*Technique work. Choose one thing to focus on and do a combination of drills and regular swimming where you work on that skill. For example, bilateral breathing, rotation, catch.
*A continuous swim or long repeats where you vary the pace or stroke. For example, 2-3 X 400m (or 3-4 X 300m) and within every 100m, do the first 25m hard sprint pace and the next 75m at "endurance" (i.e. 1500m Oly effort) pace. Or, for extra fun, do the first 25m butterfly, or the first 100 of every 400 as an IM.
*Build swims. For example, the above workout with each 100m of a 400 faster than the one before. Then a short break and start the cycle over.
*Strength/speed training (assuming your basic technique is good and no injury concerns). After an easy warmup, focus on pulling with paddles, butterfly, and short sprints at top speed.
Hope this is useful!