Al Wakra beach is the best bet for you. I live near to it and love it very much since it is the only beach where I can go easily to soak myself in.

Um Said is another one but a bit distance from Doha City. You will need to drive for about 30 minutes to 40 just to get there. Soon, once you have acquired a car then you should consider Um Bab beach. It is situated a bit far from Qatar on the west but the place is worthy for sunset view with its buble coastline and few group of spectacular palm trees. Splendid for a romantic outing!

Anyway, good luck chap!