Having had both great and terrible work out days, I found what led to the good, enjoyable days was purely the mindset. HIIT on a stationary bike sucks, but I had some fun sessions when I turned it into a game. Weight lifting routines can be super boring, but changing from rep based to time based and seeing if you can crank out a few more reps without sacrificing form can make it fun.
It takes a lot to get into that mindset for me, but it's possible and it makes a world of difference. Gamification of any task can introduce a challenge and give oneself a better purpose in the moment.