The easiest way to arrive in Tournai is by train. The main hotels are around a kilometre away from the station near the Cathedral and Belfry. It's a pleasant enough walk, but there are also frequent buses.
The local buses in Tournai have route letters - the longer distance buses have numbers. Any bus with a route letter will take you between the station and the Cathedral (but make sure you head in the right direction - especially if you're British and used to traffic being on the other side of the road). Route V goes a slightly longer way round but they all get there in a couple of minutes.
You can buy your ticket from the bus driver. (The Cathedral is called Dôme and the station is La Gare)
Coming to Tournai from London, the Eurostar would definitely make more sense than the plane!
No need to go to Bruxelles, some of these trains also stop at Lille, which is just on the other side of the French border.
The only caveat is that you would have to walk from the TGV station (Lille Europe) to the regular station (Lille Flandres) but that's a very short walk...
From there you should be able to find a regular train to Tournai (22-26 min).
Not far from Tournai, there is a small airfield where tourism planes and gliders can take off.
Located in Maubray, the Tournai Royal Air club is not really easy to find by car. Close to the Nimy-Blaton canal, this is however the perfect goal for a cycle tour. The small terrace is a perfect stop: you can enjoy a beer watching the plane taking off and landing.
The future of the site is however unsure. The land belong to the prince de Ligne and he currently plans a mega project of artificial ski slopes instead…
Not too sure if I should have put that tip in the sport or transportation category. If you like cycling, you will be happy in Tournai.
The surrounding countryside is wonderful (well, I’m maybe slightly biased here) and imho is best discovered with a bike! There are lots of small roads going through charming small villages or simply fields and pastures. If you chose this way, be aware this is not always as flat as the North of the country.
If you prefer the flat and dedicated cycling paths, this is possible as well!! Tournai being crossed by the Escaut/Schelt River, you could easily cycle along the water. There is a cycle route coming from Flanders and going to Mons called Ravel. Check their website for the different maps and look for the Ravel 1 route.
The first station in Tournai was open in 1842 by Leopold I, King of Belgium. It was located close to the bridges of the hole. The actual station, indeed the 3d one, has been open the 24 of August 1879 by Leopold II, the next Belgian king.
It is obviously still in use and you can find many connections to all major Belgian cities.
For a first contact with the city, the park in front of it could look quite poor! It was full of beautiful old trees some years ago. Unfortunately the same tornado that hitted so badly the cathedral in 1999 also brought most of these trees down!
It's not obvious to park in the center of Tournai. Worst case scenario, you will always been able to find some places close to the boulevards: around the station, the Maison de la Culture, the commercial center 'Les Bastions', in the area around the Pont des Trous.
If you want to park closer to the center, classical parking would be along the river Escaut, the Grand Place, Place Reine Astrid or Place St Pierre. Unfortunately you'd be very lucky to find an empty place on a saturday evening.
Less known parking would be close to the Grand Place on Place de l'Eveche or the Reduit des Sions. Another possibility during the evening would be the parking of the town hall from the Rue St Martin.
Tournai is very close to Lille and the French border and, even if it is not a transportation hub it is not hard to get there. This is Western Europe after all.