Posts like these come up all the time, so I'll do my best to give some advice though searching for old threads may help you a ton.
The best part about Fenway is just walking around the neighborhood on gameday. Get there early, eat a sausage, grab a beer at one of the many establishments in the area. There is also a place called The Bleacher Bar that is built into CF and provides a great view of the stadium. It's too busy right before a game, but if you go for lunch before a night game, you'll have a blast.
As for the rest of Boston, just pack your walking shoes. It's a great little city and you can see most of it in one day. I'd recommend you walk the freedom trail (it sounds corny, but I always love doing this because it gets you to see some of the great historical places of Boston), grab a slice of pizza in the North End (Ernestos) and go to the Union Oyster House.