I would suggest to search this site with Brussels or Madrid. There have been countless threads on those two cities.
However, if you’d define the lengths of the stay a bit better, I think several here could give you better ideas of what is doable. Both cities will be pretty easy to navigate and cause no great problems to a group of young people. For most in Madrid, spending two days around the broad area around Salamanca/Castellana (from Bernabeu stadium to Atocha) plus the Grand Via and Puerta del Sol is ideal. Add a couple of museums for interest, and that should take care of two days.
A similar pattern applies to Brussels. Stay around the Gare Centrale and the downtown area (Grand Place, Fish Market, Place St Gery, La Bourse, etc should suffice. For an experience, have them go to Place St Catherine and have lunch on the street at Noordzee accompanied with the best “French” fries from Decorte. The truck should be parked across the open air restaurant. It is a TOP restaurant in Belgium despite its democratic look. Around the area, one will find the original Pain Quotidien. Drinks around or on the Grand Place are great. Check the Roy d’Espagne or Cirio for a trip down history. Google Grand Place and there will be tons of restaurants (many are tourist traps) and bars. Chez Leon serve great Belgium fare but the best months are not the summer for mussels.
They should try the sandwiches, waffles, crepes at the place I linked above (across the Ibis) – they cannot go wrong with that.
For the gourmet, head to the Sablon area (Sand Market) and enjoy chocolates and other delicacies. All of this is within walking distance. The Manneken-Pis is forgettable.
If they are into comic books, Brussels is the world’s capital of the genre. If they are interested in architecture, they should check the Art Nouveau and Deco with names like Horta. There are a number of easy to navigate museums but that may not be their cup of tea. And tea should not be high on the list as Belgium is easily the best country in terms of beer and chocolate.
All in all, it would take a Herculean effort to have a bad meal and a bad time in 24 hours in Brussels, and especially for teenagers. Paris and Brussels will deliver the best food and will serve as a nice comparison to other cities in Europe, where good decent food is not that easy to found.
For one day, I’d stick to eating well and have a good time at one of the many terraces. I am not sure that kids should be doing much more than that!