We stayed just outside Rosas on a camp site a few year back. Rosas was really quiet, a big beach but very few people on it. I don't look for nightlife but I don't recall seeing many lively bars or pubs there.

If you are driving down then I do recommend that you leave the A9 at around Perpignon and go to the coast at Argeles sur mare. For there take the coast road into Spain. Great views and very little traffic compared to the autoroute/autopista crossing which can get busy at weekends. The coast road will take you nicely round to Rosas.

If you like looking at the money take a trip to Empuribrava where all the houses have their own yacht docks and some have helipads on the roof! You can hire an electric boat and cruise round the town.

Just re-read your post and noticed you are going the other way. I would still take the cvoast road however. Whatever you do avoid crossing the border by going through La Perthus. Huge queues of cars blocking the town.