Hey Bubs. FH and I took a pre-wedding honeymoon to Karon Beach in Phuket a few weeks ago. It was brilliant and I can't recommend it highly enough. We originally wanted to go to Koh Samui but decided we would take the more adventurous Phuket instead. I've heard Koh Samui is beautiful and very very relaxing though.
The things we did in Phuket:
Visited the Big Buddha - I believe there is a beach in Koh Samui called Big Buddha Beach where you may be able to see the Buddha in question
Phuket Fantasea - Fantastic stage show. highly recommend it. Even FH liked it and he HATES stage shows. Only goes for half hour or so.
Old Phuket Town
Patong & Bangla Road - beware the street sellers at night and the children. Aussie Bar is brilliant and the Margarita Bar.
James Bond Island Sea Cave Canoe - Very highly recommend this. I'm positive you will be able to do this from Koh Samui as well and it is well worth it. I'm claustrophobic, hate boats and water and still REALLY loved it. Also do Phi Phi islands. We didn't get too as the stingers were on mass while we were there.
Get a massage - They are awesome and soooooo cheap. Same with the manicures and pedicures.
Elephant Trekking - Brilliant fun but I recommend a half day trip or 60 minute trip as your bum gets sore.
We also hired scooters for three of the days we were there but beware......it is scary and I recommend ONLY do this if you have some form of history riding scooters or motorbikes. The last day we hired, we had the manager of the pool from the hotel we were at take us around. We had a ball with him and he saved us heaps of money with his brilliant local knowledge.
We also did the three island lookout, Siam King Cobra Show (AMAZING!), the Monkey school show. Also make sure you visit the pearl and jewellery megastores over there. I got two pearl bracelets and two sets of earrings for $18!!!!!!
Not sure how much of that you will be able to do from Koh Samui but I'm sure you can do the highlights. We made a pact to go back every year as the people are just amazing - brilliant service, very helpful and sooooo cheap. And it will definitely pay off to try and learn and speak their language as often as possible, even if you do butcher it like we did. lol.
Have a ball!!!! And make the most of it!!!!!