We chose to go to Koh Lanta because it's more quiet and pristine beaches than Railay area. In Koh Lanta, there are many nice and beautiful beaches such as Kaw Kwang Beach, Klong Dao Beach, Pra Ae (Long Beach), Klong Khong Beach, Klong Nin Beach, Klong Jark Beach (Waterfall Beach) and Ba Kan Tiang Beach.
If you go to Ba Kan Tiang beach, you may be subject to national admission fee as it is a national park. It is sometimes included in the tour price or excluded, depending on the conditions of the tour agencies. This place you would see the nicest beaches in Koh Lanta and you could see dragon komodo if you are lucky! Make sure you should not feed them because you never know if you might be bitten accidentally. The tour normally ranges from $60 to $80 depending on where your hotel is. This is a one day tour but it gives you enough time to relax and enjoy the nice pristine beach in Ba Kan Tiang. Lunch is normally provided in the tour price, but it is better to check with the tour agency before you consider to buy it.
As mentioned above, there are many nice beaches in Koh Lanta, we stayed in Klong Klong beach because it is more quiet but the disadvantage is there are too many rocks close to the shore. That means you could not even swim but you could see the nice sunset. If you look for the cheaper price of hotel stay, this may be your choice.
After riding the motorcycle around the beaches, Pra Ae (Long Beach) is the best and beautiful beach after Ba Kan Tiang Beach. This is the good place for swimming. There are many hotels situated in this beach but the price may be higher.
If you know how to ride a motorcycle, I would suggest to rent a bike. We paid $12 per 3 days, so it is easy to get around the island.
To get there, if you depart either from Bangkok or from Phuket, you have to make sure you arrive in Krabi. Normally, it depends on the airlines. If you take Air Asia (low cost airline), it normally takes you to Krabi (Lanta) with the shuttle bus service with free of charge. If you take other airlines, you may have to take a bus, a van or hotel car. Normally, you will get this from the airport but it may be a bit confusing for some people. Unless you prefer paying $80-$100 from Krabi airport (if taken other airlines except Air Asia), someone will pick you up at the airport.
Tips: Koh Lanta may be a bit more quiet than Railay, so some areas may be conservative due to Muslim community. If you want them to respect you, please make sure you wear your pants at knee length (This is my observation!).