I like happy songs more than motivator songs, but "Knee Deep" by the Zak Brown Band and Jimmy Buffet is a great one, "It's Good to Be Us" by Bucky Covington always makes me smile, and "Everything is Fine" by Josh Turner is great. I'd recommend them for weightlifting or stretching more than for cardio though, unless you're a super chill runner like me.
Re: Gym playlist