Choose something that scares the h3ll out of you and forces you to face your innermost fears. If you're not ready for that, then look for something that requires focus, attention, and if possible, interaction with other people.
I started doing home improvement projects. I know that doesn't sound scary, but I took on a few things that, if I'd screwed up, would've been very costly to repair. And, with all of the saws and power tools, I had to pay attention or risk losing some fingers. I also had a lot of interaction with people (the Home Depot crew and all of the neighbors stopping by to see what I was doing).
Try some things; anything. The key to success is to find something that really motivates you, makes you happy and keeps your mind off your marital issues.
