Pruning/wiring bonsai trees.
I don't like all the laborious effort that goes with repotting 150-200 trees annually, but it's essential to their good health. I find pruning and wiring a wild looking mass of branches and foliage into something that manifests my vision for what the plant should look like in the future to be both extremely rewarding and relaxing. I get so engrossed in the pruning and wiring that it isn't unusual for my wife to come out and remind me it's almost 10PM and dark, and I haven't had dinner yet.
Al