Hostal Olimpia 

Great for the low budget, crisis afflicted couples. Double bed room, with private bathroom, 9 days 360 €. Right on the on bus lines (L96,L97,L95 go the beach, L99 goes the airport) which makes getting around Castelldefels very easy and convenient. The staff is awesome and sweet. Grocery store right in front of the hostel, a fruit store on the corner, 5 mins from the Renfe, and 2 minutes to the center of town where you can find all kinds of eats. Although, buy earplugs if you want good nights sleep- very noisy, loud up until 2 a.m. and restarts at 5-6 a.m. with a garbage services passing by.