I'm late to the conversation, as I only just started STS for the first time last month. After completing Meso 1, my first rest week was last week and I chose to do some light yoga, go for casual walks or a casual bike ride, and that was the extent of activity. I took "rest/recovery" at its word, so opted not to do anything especially vigorous. I also treated myself to a Thai massage (they will assist you with some stretching), and also swung by the chiropractor for a "tune-up". I felt totally great and was ready to start up Meso 2 this week feeling completely restored.
Not sure I'll manage a massage and chiropractor trip after Meso 2 (gets kinda pricey), but I'll be seriously considering it when I'm nearing the completion of Meso 3.
Rest and recovery are important to me. I imagine my taxed muscles taking that week to fully heal and repair. May be more difficult for them to do that if I'm asking them to bounce around to intense cardio.