Went to Alghero back in June and loved the place, though I have to say I'm not 18 years old! There were loads of places to both eat and drink in the old town [I'd recommend staying as close to the old town as possible - there are two hotels within the old town, they're fairly basic but full of charm check out here
http://www.mamajuana.it/beb_eng.asp and the other one, which I prefered, was the San Franceso hotel - can't find it on google but have the web address at home] ranging from expensive sea food restaurants to basic pizzerias plus loads of cafes and bars, even a couple of pub style bars, however it's not exactly packed with nightclubs if that's the type of holiday you're looking for. In terms of activities there's lots to do around Alghero, there are some amazing beaches [though not really the ones closest to town - we went a bit further north to Porto Conte which had excellent beaches and I belive there are plenty south of Alghero too] and the area is a Mecca for divers. There's boat trips you can take out to the Neptune's grotto series of caves and if you hire a car it definitley worth heading up to Stintino for La Pelosa beach - the rough guide to Sardinia has loads of information, but I guess it depends exactly what type of holiday you're looking for. I can't recommend it highly enough - but then that's just me.