you can definitely do it. haven't done so myself there so someone else will hopefully help.

Yes you can I did it at the Na Muang waterfalls (sp) just inland from Lamai , this is widely advertised around the island

Yep, just got back from Thailand and was in samui a couple of weeks ago - we did a half hour elephant trek Na Muang waterfalls, worked out about £10, don't pay for a photo tho - the guys who take you round offer to take loads with your own camera then just charge you 200 bahts - still a bit of a rip off but he took about 10 really good pics of us so we didnt mind!
If you can do the day trip round Samui - it only costs around 400 bahts for the day and the elephant trekking is all that's extra - really good value and literally covers everything there is to see on the island.
Marine Park trip is def worth it too - hope this helps

when you are over your jetlag I'd love to hear how you got on