Koh Chang is more useful if you like to have a quiet and relaxed beach vacation. There is a regulary Baht Taxi Service. They drive the entire day from the north to the south of the island. As well there is an Elephant Camp and a Waterfall. There are two bigger supermarkets (Macro abd BigC) just behind White Sand Beach. WSB is by the way the best location to stay. And I can recommend the KC Grande Resort. There are planty of Pools as well as a nice Gym and awesome beach.
I bought in the meantime 4 motorbikes in Pattaya, 2 on visa exemption and 2 on Non/O visa and had never problems to get the residence cert. A nice lady in a motorbike rental shop always doing for me the yearly tax/insurance cert. as well as provide me with residence certificates for all reasons. Cost me just 300 Baht service charge.