Fartlek workouts can add some intensity to your program while freeing you from some mental stress of a structured interval workout. Fartlek (which means speed play) sessions consist of repeated accelerations to a faster intensity with periods of easy running in between. In this data obsessed era I'm often annoyed to see precisely measured sessions, in terms of time of effort and recovery, being called fartlek. The point is the "play" and these sessions should not structured or programed into your Garmin. The efforts can be done at will to physical markers (big tree, lamp post, building etc.) and your body dictates when you're recovered for the next one.
I like warming up for 15 minutes, following this with 20 minutes of efforts anywhere between half a minute to three minutes followed by a 10 minutes cool down. Leave the watch at home and sprint between landmarks on your favorite route. The same concept can be applied to your swims and rides. It's also a good workout to do with a buddy, taking turns to pick out a finish point for each effort, and waiting for each other during the recovery periods.
Keep yourself physically ready and mentally fresh by adding this occasional unstructured speed session to your endurance work.