As I usually arrive in the evening, I like going to Piccadilly Circus the first day, (generally, the operating time for trains is from 5:00 am until midnight, Monday to Saturday. Opening hours are reduced on Sundays, the timetable can be checked in http://www.tfl.gov.uk/modalpages/2625.aspx) make some pictures and take a walk by this area.
From Piccadilly we can go and visit Chinatown (you can’t miss taking some pictures to the entrance arches and to the “small animals” hanging on the windows – ducks?).
We are beside Soho (gay scene area in London. We can also find a lot of sex-shops where you will certainly find that item related to sex that you had been looking for for so long. Any consumer good related to porn has its place in this neighborhood… Don’t be surprised if you are offered prostitution services on every corner).
For dinner: there is a couple of Italian restaurants by the area with a good quality-price rate.