The beauty of the Marlin 336 is that you mostly take it out of the box and use it. The only upgrade I would do is to change the sights. Either add a scope or a good peep sight, and if your eyes are good I'd recommend a peep over a scope by a good margin so as not to mess up the handling.
Just MHO, but a lever action is not a long range bolt gun and doesn't benefit all that much from treating it like one. It's a great handling, reliable hunting rifle. You can do the trigger, you can (as I've done so often) fiddle with the barrel bands and such to try to accurize it. That's fun if you're into such, but 336's generally don't need it.
What caliber were you thinking of getting?
