Belgium in 2-3 days
Take the Eurostar to Brussels and explore this city's medieval core. Sit down for a beer in the main square then head around the corner to be underwhelmed by the famous Mannekin Pis, a statue of a little boy pissing (seriously).
Explore the cobbled streets around the Grand Place and browse through the chocolate shops. Be tempted by the many touts out the front of their restaurants in the streets around the square. Haggle with them to get a free drink with your three-course meal.
After a night in Brussels you should take the train to Bruges. Spend two days wandering the streets, cruising the canals and sitting in cafés eating chips and mayonnaise — washing it down with a local brew of course.
In 7-9 days
With more time to spend in Belgium spend longer in Bruges and then try to see Antwerp and Ghent as well. Head out to the coast and take a trip on the Kusttram — a tram that trundles down the coast from Knokke to De Panne.
Head inland to Ypes — 30,000 Allied soldiers were killed here during World War I and the area is dotted with cemeteries and memorials.
Two weeks +
If you're in Belgium for more than two weeks you could rush around to see more of the country — heading east en route to Luxembourg — or you could just take your time seeing the main sights, on a bike! Belgian's love cycling and Flanders is a nice flat area. If you're keen for some uphill action head to Wallonia for some mountain biking.