If I do my long run on Sunday, I'll take Monday as a rest day. But if I do my long run on Saturday and take Sunday as a rest day, I'll use Monday as a strength-training day. I prefer to do this in a class format because it forces me out the door, and I am paying someone to motivate me. I usually take a double-floor Barry's Bootcamp class (Monday is arms and abs), a Fhitting Room class, or an Orangetheory class. (If I don't go to a class, I'll do a full-body strength session at my local gym.)
I like Fhitting Room because it's structured as a HIIT class with different circuits split up into set time to work and set time to rest. A lot of the class is strength-based, using equipment such as dumbbells, kettlebells, and TRX straps, but there are also intervals with cardio equipment to keep your heart rate up (assault bike, SkiErg, rower.) I also like taking an Orangetheory class because it includes HIIT blocks of running, sprinting, and walking for 30 minutes and 30 minutes of strength training. With Orangetheory, I get a cardio and strength workout done all at the same time.