Saturday get an early start before 8am and visit Portobello road. While the market sets up have some excellent coffee and pastry at Gail's Artisan Bakery. Its a lovely 10-15 stroll from the Notting Hill Gate tube station to it. At around 8-8:15 start a leisurely exploration of the market. Take a good 2 hours. Have lunch there.
Then hop on the tube at Ladbroke Grove and take a 25 tube ride to St Pancras/kings Cross station. Be sure to stop to see the Lovers Statue ( cant miss it really) as well as the statute and tribute to the man who responsible for saving the station from demolition in the 60's, Sir John Betjeman.
From here walk to the nearly adjacent British Library, enjoy the issac Newton statue then visit the Treasure Room, a.k.a. The Sir Lohn C Ritblatt gallery. Check the library website to see if they'll be serving afternoon tea as well as what lecture, event, etc might be occurring that day.
If the the above generates no interest I suggest a walk along the Thames starting at Parliament Square. Stroll across Westminster Bridge and head toward the London Eye. Follow the foot path all the way up to the Shakespeare Globe theatre. . If you like modern art enter the Tate Modern. If not walk across the Milenieum bridge. From there either walk along the river back towards Blackfriar tube station or walk along Queen Victoria street to the Blackfriar Pub and have lunch. From here walk back to St Pauls and then on up to the Museum of London.
Lastly, another interesting option is Brick Lane and Spittafields Market. I'd tube to Aldgate East, exit, walk up to Brick Lane and walk to Suzzle for some excellent early A.M. Coffee. Proceed from there to Brick Lane and explore. Then off to Spittafields and have lunch.
Leaving what to do Saturday evening up to you.
On Tuesday visit the Royal Academy of the Arts or the Photographer's Gallery off of Oxford Street. If you chose the gallery its a 5 minute walk to Liberty which is a very unique department store.
Enjoy your quick return visit.