Which is correct: "It's a beautiful day. I am not taking/won't take the bus to work today. I prefer walking."
Both are perfectly correct. If you're doing homework then I'd say the second way with "won't take" is the most correct since it's probably trying to teach tenses, however in reality I'd be more likely to say the first way.
Another way to say it would be "I'm not taking the bus today since it's such a nice day out, and I prefer walking anyway."