World Heritage sites
I've seen a couple of people doing this: go thorugh the list of UNESCO world heritage sites and note the ones you have been to. I count 83, so only 828 more to go... rather shamefully, there are two in Brussels (this and these) and one in Antwerp that I have not seen. It's just as well that so many old capital cities are on the list or my tally would be considerably lower...
Austria
Azerbaijan
Belgium
Bosnia and Herzegovina
Croatia
Cyprus
Czech Republic
Estonia
Finland
France
Georgia
Germany
Greece
Holy See
Hungary
Ireland
Italy
- Historic Centre of Rome, the Properties of the Holy See in that City Enjoying Extraterritorial Rights and San Paolo Fuori le Mura
- Historic Centre of Florence
- Piazza del Duomo, Pisa
- Venice and its Lagoon
- Historic Centre of San Gimignano
- Historic Centre of Siena
- Early Christian Monuments of Ravenna
- Archaeological Areas of Pompei, Herculaneum and Torre Annunziata
- Costiera Amalfitana
- City of Verona
- The Dolomites
Jerusalem
Kosovo
Luxembourg
Macedonia
Malta
Montenegro
Netherlands
Portugal
Russian Federation
San Marino
Spain
Switzerland
Turkey
Ukraine
United Kingdom
- Giant's Causeway and Causeway Coast
- Stonehenge, Avebury and Associated Sites
- Blenheim Palace
- City of Bath
- Frontiers of the Roman Empire
- Westminster Palace, Westminster Abbey and Saint Margaret's Church
- Canterbury Cathedral, St Augustine's Abbey, and St Martin's Church
- Tower of London
- Old and New Towns of Edinburgh
- Maritime Greenwich
- Royal Botanic Gardens, Kew
- Liverpool – Maritime Mercantile City