Beijing doesn't really have an off-season; it's just that it's much less crowded when it's not a public holiday. If it's your first time visiting Beijing, I'd still recommend finding a small tour group in advance and making sure you understand the daily itinerary. It's best when you can visit places like the Forbidden City, the National Museum, and others without having to worry about any of the planning – a completely hassle-free experience. Otherwise, you'd have to make reservations for many attractions yourself. With a tour, your meals, accommodation, and transportation are all taken care of. When you arrive at the sights, the driver and guide will sort out tickets and provide commentary. That's what my friends and I did.