score:16
I'd say Lille (in Belgium they might call it Rijsel.)
You can get there in 39 minutes with the fast trains and if you are lucky you do not need to pay more than 19 Euro, but 25 Euro is more likely for each leg (prices 2017.)
Those fast direct trains run hourly at least, there are also normal (less fast) trains which have no restriction on the number of tickets sold, but might require a change of trains.
You can find this information easily on the site of the Belgium railways.
To see just the direct trains, select advance search and change the show all trains to direct trains only.
Other options, (longer travel, but not hard) are Amiens and Arras.