Looking at Rick's Daily Reminder section of his London book, I see the following:
Monday: the Houses of Parliament may be open as late as 22:30
Tuesday: the British Library is open until 20:00, and the Houses of Parliament may be open as late as 22:00. On the first Tuesday of the month, Sir John Soane's Museum is open until 21:00
Wednesday: the British Library is open until 20:00, and the Houses of Parliament may be open as late as 22:00.
Thursday: the British Library is open until 20:00, and the National Portrait Gallery until 21:00
Friday: Sights open late include the British Museum (selected galleries until 20:30), National Gallery (until 21:00), National Portrait Gallery (until 21:00), Victoria and Albert Museum (selected galleries until 22:00), and the Tate Modern (until 22:00).
Saturday: The Tate Modern is open until 22:00
I agree that theater (sorry, theatre) is a great thing to do in the evening. There is a tremendous number and variety of shows on at any time - you're sure to find something of interest. There are also movies - London has a greater variety of both current films and revivals than almost anywhere else.
Besides the London Eye, there are other great viewpoints, some free, some not; some open in the evening, some not. Again, Rick has a list on pages 90-91 of his London book.