Ao Nang is a more family oriented place for a holiday in Thailand and unlike Phuket or Pattaya, there are basically only two relatively small "red light districts" in Ao Nang.
On the other hand, there are many restaurants, pubs and disco like bars along the beachfront in Ao Nang and neighboring Nopparathara Beach. Plenty of vendors set up their tables in the evening and you can enjoy shopping for souvenirs or have a beer in one of the pubs after your dinner in one of the restaurants.
As a very special and cozy place to sit near the beach in the evening I would recommend the Last Fisherman Bar, just behind the Golden Beach Hotel, right on the beach. Every night, they have a BBQ and you can just sit there and watch the sunset, listen some decent music and enjoy the fresh sea breeze. Maybe some unforgettable evening.
If you are looking for Life Music, there are several places along the main road, most of them at the north end of Ao Nang, between the L Hotel and the main entrance to Krabi Resort (Echo Bar, Relax Bar, Full Moon Bar and Moo2 Bar)
At the other side of the beach road, you have to walk up (or take a MB Taxi) for about 500 meter from the beach, pass the MCD Restaurant you will find at least 2 bars with live music (Reggae Bar).
Another one, the Luna Bar has Live Music sometimes but is located around the corner and already on Nopparat Tara Beach.
Other popular places to hang out in the night are the Center Point (Bars & Restaurants), and the famous Soi "RCA Entertainment" with the typical Beer Bars.
At about 10 to 11 pm it gets quiet already in Ao Nang, only the bars and some pubs stay open for the youths and others who like to party maybe until sunrise.