i love the national gallery. i also love the british museum. it's my favorite museum in the world. A History of the World in 100 Objects is a great read, almost a prerequisite, before going to the british museum. it highlights 100 of the most important artefacts at the museum.
something not mentioned in this thread yet- st martin in the fields. it's right off trafalgar square, across the street from the national gallery. get in there for one of their concerts. they have loads of them. my favorite ones are the lunch concerts. they're cheaper than the night concerts and you don't have to make reservations. if you do make a reservation for an evening concert there's a bistro in the crypt that's alright but if you're feeling adventurous make a quarter mile trek or so north of the church on st martin's lane towards leicester square and see what suits your fancy.
i've seen it briefly mentioned twice but i want to enforce the recommendation for going to greenwich- the national maritime museum and the royal observatory are there. tremendous museums which could easily both fill several hours of your day. i wouldn't recommend going to both in one day as they're both informationally dense.
sunday roast - pick a spot and make a reservation.