Mount Lavinia BeachThe beach that is closest to Colombo, The commercial hub of Sri Lanka. This is usually crowded, especially on weekends. The area is famous for its sea food restaurants, and tourists regularly visit them when they are in Colombo. There are many hotels in the area, with Mount Lavinia Hotel being the most famous one. Unless there is heavy traffic it is only around 30 minutes drive from Colombo city.
Bentota BeachAround two hours drive from Colombo, Bentota beach is ideal for tourists who don't want to travel too far from Colombo, but still wants to spend a relaxing day or two by the beach. The beach is not crowded and perfect for sunbathing. The sea is a bit rough in the area so it is advisable to check with the hotel whether its safe to go to the sea. There are few hotels in Bentota with Taj Exotica considered the most luxurious in the area.
Beruwela BeachAnother famous beach that is near Bentota.These two beaches are so close to each other sometimes they are referred as the Beruwela-Bentota beach area. Here also you can spend a relaxing holiday in a relatively secluded beach. The sea is a bit rough but if you can catch a boat ride to the nearby island it has a great beach area ideal for swimming. Price is relatively cheap compared to Mount Lavinia and Bentota.
Hikkaduwa BeachHikkaduwa is famous for beach parties and it is one of the favorite beach destinations of the local young crowd. Also Hikkaduwa has some of the best coral reef in the island and it is a great place to go snorkeling. You can view the corals by boat rides too, where they have glass bottomed boats. Since the beach is usually crowded it is not ideal for a relaxing holiday, but if you want to enjoy great sea food and have fun mixing with the locals then this is a great beach to visit.
Arugam Bay BeachConsidered one of the top ten surfing destinations in the world, this is a beach frequently visited by top surfers. Since this is to the east of the island away from Colombo you want find much of a crowd even during the weekends. As mentioned above the main focus is on surfing and there are plenty of surfing equipment to buy if you are in need. Also if you want to try your hand at surfing there are surfing lessons as well.
Negombo Beach areaAnother beach are that is close to Colombo and even more closer to Katunayaka international airport. Tourists who are visiting purely for the sea bathing and sunbathing find this a very convenient beach area.There are lots of hotels and restaurants catering especially for tourists. Negombo is famous for sea food, especially various dishes prepared using prawns.
Nilaveli Beach areaOne of the most purest white sand beaches in Sri Lanka. Inaccessible to locals for almost twenty years because of the civil war, people started flooding to the place after the war. The beach is safe for sea bathing and the nearby pigeon island is a great place to go snorkeling as well. Although it is very distant from Colombo you will find the beach crowded, especially in the weekends. However hotels have private beach areas where you can spend a relaxing holiday. There are few hotels and more are coming up to satisfy the demand.
If you feel more beaches need to be added to the list feel free to mention it in the comments section.