DO NOT SKIMP ON FOOTWEAR! Vacations abroad are by their nature walking vacations. The center of many towns are traffic free so walking is how they are seen. Buy quality, well fit, supportive and comfortable shoes. Make it a priority. Buy them well before you leave and walk in them often. Blisters or sore feet can ruin your time. The only way to avoid this is preparing ahead. Be well prepared with the best socks and footwear you can find. Buy good socks to go with your shoes. Steer clear of cotton. Cotton looses it's cushioning abilities quickly and takes a long time to dry. Wool or synthetic blends continue to cushion even when wet and the are much easier to rinse out and wash. They dry more than twice as fast, because they breathe well and wick moisture.
Some of the most difficult (tiring) city pavements/terrains for walking: Jerusalem, Nazareth, Rome, Venice, Florence, Siena, Assisi, Athens, Thessaloniki, Cuernavaca, Mexico City, Old Quebec City, Dublin.
Such a simple little tip, but worth so much! Trust me. Your feet will thank you.