I'm headed to Costa Rica with my girlfriend next month for 7 days. I've been to Tamarindo and Montezuma before and want to visit somewhere new.
I'm considering Mal Pais but I've heard you need a car top get around because it's really spread out.
What about Dominical?
What other beaches would be good for a 5 day stay? I wont have a car so I'm looking for somewhere that is easy to get around by foot with some moderate nightlife.

You hard right about Mal Pais - everything is spread along a 3-4 mile stretch of beach. You could get around on a bicycle during the day but it would be handy to have a car at night.
Samara is small and easy to walk around but you might get bored there after a few nights
What about Manuel Antonio?

Cahuita and Puerto Viejo are definite options if you want to go to the Caribbean side. I think you would enjoy the central pacific area as well. We live in Esterillos Oeste which is only 15 minutes south of Jaco. So it is close enough if you want some night life in Jaco. The beaches in and around Esterillos are great. There are numerous small little bars and restaurants along the beach and the surf is great for boogie boarding and surfing. You can walk for miles in both directions on the beach. There is plenty to do as far as activities, but it's also a great place to just relax.

Check out CostaRica.com for national weather service data by town.
You will find that the best weather in September will be found in the area around Puerto Viejo de Talamanca in the South Caribbean. (Check out Aquamor and the What to Do or Things to See sections in the hotels listed).
The Puerto Viejo area is a great place to take your girlfriend. There is a gold sand beach about 1.5 km South of PV near La Costa de Papito, and a romantic setting too. Bring your sunscreen! Enjoy