Take note though, that it is only the short distance buses, doing the ‘town service’ within the city center (the ones that are painted white) that are for free. The yellow ones, doing longer distances, demand fares.
There is also the option of using the train. The train will, however, only get you to Mons and around a few places in Mons.
Further on is the option of hiring a car, and using it to get around Mons. This is not a very clever move, because for one, Mons is a smallish city: the sort of city that doesn’t quite justify being explored on a car. More importantly though, you will come to realize that there are places where cars are discouraged from going in Mons, and most of those are actually the sorts of places that would be of most interest to you as a person looking to savor the awesome sights Mons has to offer.
Being a small city, Mons can be quite extensively explored by foot, and this is indeed a worthwhile adventure. The cobbled streets you will be doing most of the walking on are just amazing.
If you are a keen observer, you will notice, as you try to get around Mons, that many of the streets that encompass the city are actually built where the fortifying walls of the city once stood. The experience of being there is just surreal, as are the sights you come across.