Getting new coral is by far my favorite, but I'd say that looking at my tank top down is the next favorite activity and then comparing growth pics. Once a frag has grown out, you can really see what all of the hard work and money was for.
It's gotta be the livestock selections that you make for your own slice of the ocean growing in or becoming comfortable. It's a satisfying feeling seeing the animals in your care grow, find their favorite spots, and develop their personality. Watching green slimer split and sprout has been rewarding. Thing is starting to grow like a weed even in high nitrates.
Starting a new build is fun too. It's a blank canvas and it's hard not to imagine 2 years from then and what it will look like.
Mine wasn't on the list. I love trimming corals. It is like when I pick fruit off of the tree in my yard. It makes me feel like my work was not for nothing.