Walk the island to see Notre-Dame's exterior and the flower stalls; great for an easy, romantic first-date stroll (interior access may be limited during restoration).
Spend the morning seeing highlights (Mona Lisa, Venus de Milo) — Louvre typically opens around 9:00am and is closed on Tuesdays, so plan accordingly; prebook tickets to skip lines.
If you want the classic Paris moment, head to the tower for city lights — elevator hours vary but the tower is usually open late; prebook the summit if available.
Explore the basilica (open early to late day, free entry) and the winding streets around Place du Tertre with artists at work — incredibly picturesque and intimate.
See Impressionist masterpieces in this beautiful former train station; typical hours are ~9:30am–6:00pm and the museum is often closed on Mondays (check before you go).
Buy pastries/sandwiches from a nearby bakery and picnic in the gardens — relaxed and romantic with statues, fountains, and people-watching space (gardens open all day).
Stroll Place des Vosges, browse boutiques and galleries, and sample falafel in the Jewish Quarter; Le Marais is lively and historic, great for relaxed exploration.
Walk a nearby neighborhood (Saint-Germain, Marais or Île de la Cité) to pick up souvenirs, press-your-flowers, or a small gift — the low-pressure finish to a first-date trip.