I followed most (mainly the theoretical) courses of the nanomat master and I think the answer to your answer depends on what you're looking for. As it is a master programme, the quality of it depends on the research that is done by the professor that teaches the particular course. As for Antwerp, we're mainly focussed on condensed matter (both theory and experiment (although also our empirical elementary particles and visionlab group are interesting)), so if you're looking for astrophysics, general relativity, string theory, you'll be better of elsewhere.
But because the nanomat programme is about condensed matter, you're quite at the right spot. The professors that give the (optional) courses use their content everyday in their research. Just click on their names and check out what research they do, if you think it is interesting, it is already a very good parameter of what to expect from these courses. About the compulsory ones, these are the standard ones that you'll find everywhere.
Answers to your questions:
1) Challenging is something you make it yourself. There are about 20 students in the total master (so for about 40 professors), so in projects/thesi, you will be pushed if you're looking for it. If you want to be challenged in problem solving, look for courses of Jacques Tempère. He's style is: no exams, but assignments by which you explore the course yourself! Very interesting, but tiresome!
2) Are you planning to do your full two years of master in Antwerp, or are you only coming for one semester (or one year)? In the former case, it is advised to take up the compulsory courses in the beginning since they are necessary for the other courses, in the latter, just shop around in the optional ones and come to Antwerp in the second or third semester after you've done all the necessary compulsory things in Jussieux. You can take up the courses in the order you prefer, so in that sense there is no distinction between first or second year, just think and plan a bit ahead and don't forget that you also have a master thesis to finish, but that's obvious.
3) So, this really depends on what you're looking for. Theoretical things from Jacques, so both the path integral ('t Hooft once called Antwerp "Mekka of the path integral", so apparently we're good at it), the superfluidity and the solid state physics course. I didn't do experimental courses, but electron microscopy is something we're also good at.
4) For the internship, this changed a couple of times since I did it (at UPMC btw
) and I know that there is some discussion about it whether foreign students are allowed to do it in the university (Belgian students are not allowed since we're supposed to leave for at least 12 credits). Check with prof. Milorad Miloscevic, the coordinator of the programme, for the details, but you could do it outside the university for sure (and they will help you find it) if you want to, but I think you could also do it inside, but I'm not sure. It's 12 credits and lasts 6 weeks.
Antwerp is a very nice city, but as everywhere in the world (except Paris), they've put the scientists outside of the centre, at about 15 min by bike from the central station. Everyone rides the bike, although public transport is also ok, but don't expect fast metros every 2 minutes, nowhere it is as good as with you. The big advantage of a bike is that you can use it also after 1 am and after some very nice Belgian beer
Voila, if you've got any more questions, feel free to ask!