beautiful, beach either in fuwairit, or next to sealine (but very crowded and too many cars) or if you're lucky and find someone with a 4x4, inland sea. corniche, forget it, you can go for a walk and that's it.
you can also try the hotel beaches but I'm not sure if you have to be a member - think the InterCon is open for non-members, but they want QR 200 - for me, that's too much for one day...or you can also go into Sealine, which is about 45 minutes drive from Doha, and costs you QR 50 per person plus QR 25 for cars on the weekends and QR 30 per person during the week.