I´m a 25 year old girl currently working & living in Koh Lanta, Southern Thailand. I would say its the perfect spot for female solo travelers, as its safe, easy accessible and well developed. Ive been here now for about three weeks and staying at least five more!
The party scene is not the most crazy, but there are some really nice beach bars with good vibe and every night there´s a party at a different location around the island. There are also plenty of good restaurants serving both Thai and western food!
There are 9 beaches around the island, and its easy to rent a scooter to get around and discover the island. When it comes to surfing I have heard there are sometimes waves in Klong Dao Beach in the green season but there are no surf shops around- on the other hand there´s a gym and some very good yoga studios.
Most nationalities can apply for a 90 day tourist visa at the Thai embassy before departure. It shouldn't be a problem to get it extended.
I think the best thing with Koh Lanta right now is that the tourist scene is slowly changing, and you can get some great bargains when it comes to accommodation (around 200 USD a month or even less is possible!)
The island also have a coworking space