I don't know if there will be an answer out there other than the one I have thought of, but when I put out questions in this community, someone always answers with at least one idea I would never have thought of, so here goes:
I am doing FB30 Round 3 right now. I've been doing a bit of travelling which has disrupted my workouts, but as a happy accident, I find taking 3/4 days off in a row means I can really push hard during a HIIT work out and do a great job (for me) keeping up and have some spare energy left in the tank for, say, an upper body strength workout. I feel awesome at the end of it. My body seems to like this format - motivation is not usually too difficult but now it's even less difficult. For 4-5 days in a row, I can't wait to work out and then for the next 3 days I get the message from my body: 'no chance' on working out and I don't mind complying.
Trouble is, like today, after coming back from 4 days away, I've done day 1 & 2 of week 6 on the same day. Day 1 is HIIT and day 3 is HIIT which means tomorrow's scheduled work out is HIIT. I know it's not recommended to do 2 consecutive days of HIIT. At the moment, I end up looking around for another workout for tomorrow to fill the gap. Having done upper and lower body in one day (today's HIIT also had lower body strength), the options get a bit limited. Tomorrow, I will look for cardio + core or cardio + whatever isn't sore (if anything)! And though the video search is fabulous, I'm lazy and I'm not fond of looking for videos, I love the convenience of a program. I can't think of another way to fill the gap though.