I'm not sure what you were doing in the gym, but for me lifting heavy and pushing the boundaries of what I can do every single time I'm in the gym makes it fun.
It also helps if you setup regular routines to make comparison easy, for example:
day 1 - bench, curl (barbell), lat pull, tricep extension (dumbell)
This example hits alternating muscle groups so you can minimize rest in between. If I do something like this I can usually get my heart rate up as well.
This example hits alternating muscle groups so you can minimize rest in between. If I do something like this I can usually get my heart rate up as well.