Tiananmen Square and the Forbidden City take a solid morning to early afternoon. If you go to the mausoleum it's probably a whole day. If you get finished with the Forbidden City and feel like seeing more, Beihei park is a few minutes walk from the northern exit of the Forbidden City. It's got a few interesting temples and sights and is easily combined with Tiananmen Square and the Forbidden City. Walking through Beihai Park is a nice way to unwind from the massive historical sights you've just witnessed.
Dinner: It's your first night in Beijing and you're probably pooped out from all the walking and sight-seeing. Don't let this stop you from sampling some traditional Beijing cuisine. Nip back to the hotel for a shower and a quick rest and then go out for some delicious Beijing Duck, it's the specialty in Beijing, after all. If you're really tired, your hotel probably has a restaurant that serves it. Otherwise, ask for a recommendation of somewhere nearby.
