If you go to Mérida be sure to acquire the ticket to visit the main attractions. For people with the youth card the ticket is only 6euros, for adults 12euros. The roman theatre is quite impressive and the arena as well; there are several points with information and it is interesting. The Alcazaba is also impressive, and it gives a good view to the roman bridge. There are other places to visit that you don't need ticket, like the roman temple and the roman bridge itself.