The battlefield of Waterloo lies some way south of the town itself - Wellington had a habit of naming his battles after the place he slept the night before, rather than where they were actually fought. The position which the allied troops held was along the ridge either side of the Mont-Saint-Jean crossroads, and the French did for a time refer to it as "Le bataille de Mont-Saint-Jean". In Waterloo itself there is a small museum in the house in which Wellington stayed.
The top of the ridge on the battlefield proper is dominated by the Butte de Lion, a 30 metre high conical mound topped by a statue of a lion marking the spot where the Prince of Orange was wounded on 18 June 1815. The mound provides a good viewpoint, but the excavation of earth for its construction altered the lie of the land considerably in the surrounding area, a much fought-over part of the battlefield, so the perspectives and the dead ground are not exactly as they were for those who fought there. At the foot of the mound a visitors' centre and a battlefield diorama are worth a look; there is also a cluster of restaurants and tourist tat shops around the point where the formerly sunken road along the ridge forks with the track down towards Hougoumont. A cluster of other monuments stand near the crossroads of the ridge road and the main highway to Charleroi.
Much of the battlefield has not changed greatly over the past two centuries; the exception is the right wing of the allied front which is crossed by the southern extension of the Brussels Ring motorway and, beyond it, built over by the suburban sprawl of Braine L'Alleud. Only outlying buildings of the chateau of Hougoumont (now just "Goumont") survive, but la Haye Sainte is almost exactly as it was left after the battle and is still a working farm in private hands. There is little trace of the sandpit on the other side of the main road, however. The Charleroi road itself has obviously become a bit wider and rather better surfaced, but follows the same line as it did. The inn of La Belle Alliance where Wellington met Blücher after the battle was a nightclub the last time I checked; Napoleon's last headquarters at Le Caillou, also on the main road, is preserved as a museum.
The allied left wing, and the area from which the Prussians emerged from Wavre, have also undergone some suburbanisation, but the relatively broken terrain in that direction is still evident.
The old main road can be followed southwards (avoiding the new bypasses) to the Quatre Bras crossroads - the route down which the allies retreated between the two battles, although the bridge over the Dyle in Genappe which was a serious pinch point during the marching and countermarching is no longer there (the river, such as it is, is in a culvert).
Would-be visitors who want a better guide than this to the battlefield or anything they will find in situ - and anybody else interested in the subject - are recommended to read anything on the subject by Richard Holmes and John Keegan (especially the relevant part of The Face of Battle) and Waterloo: New perspectives by David Chandler which corrects some historiographical errors in the traditional British narrative (which tends to ignore the contribution made by Allied troops other than the British, as the majority of eye-witness accounts made public at the time were gathered by a historian who didn't speak anything but English and whose independence was severely compromised).