Take Heathrow Express + Tube or the Heathrow Connect/Elizabeth Line to King’s Cross/St Pancras; allow buffer for luggage and security (aim to be at St Pancras by 09:30–09:45).
Walk around Notre‑Dame's exterior and visit Sainte‑Chapelle (usually open ~9:00am–5:00pm but check seasonal times); the stained glass is worth the visit.
Visit the State Apartments and Hall of Mirrors (typically open ~9:00am–6:30pm but hours vary seasonally — check before you go); allow several hours to see palace and main areas.
Explore the Grand Trianon, Petit Trianon and the extensive gardens (garden visiting hours vary; fountains or musical gardens sometimes run on set days).
Spend the morning at the Louvre (usually open 9:00am–6:00pm and closed Tuesdays — you are visiting on Monday so it should be open); prioritise a few wings to avoid burnout.
Take a Bateaux Mouches or similar Seine cruise for illuminated views of Paris (evening departures commonly 7:00–9:30pm; check exact schedule and book ahead).
Visit the British Museum (usually open 10:00am–5:00pm); highlights include the Rosetta Stone and Elgin Marbles — general entry is free but special exhibits may charge.
Head to South Kensington to see the Natural History Museum and the V&A (both usually open from ~10:00am–5:50pm); both offer world‑class displays and are mostly free to enter.
Self‑guided or guided walking tour of the university colleges, Radcliffe Camera and the Bodleian (opening times vary by college; check ahead for college entry and guided tours).
Visit the Ashmolean (open roughly 10:00am–5:00pm) with its archaeological and art collections; general entry is free but check for special exhibitions.