The old town, Vianden and the Müllerthal in a weekend
Leisure trips to Luxembourg tend to be short and full. The old town and the casemates on foot, a day north to the castle at Vianden or into the Müllerthal, where the trail winds between sandstone rocks and forest, and an evening in one of the wine villages on the Moselle. That adds up to maps, timetables for the free buses and trains, and photos: the 3 GB pack, valid 15 days, covers it without counting.
Take 5 GB for 30 days if the weekend turns into a week, or if you post video as you go. People here for work, with meetings in Kirchberg where the European Court of Justice and the European Investment Bank sit, usually need more than tourists: the FAQ below covers that case.