Thailand trip was the best trip of my life till date :). I went with my husband to celebrate our 1st wedding anniversary and enjoyed every bit of the trip. We have not booked any Tour/travel agency and did organize the trip by ourself.
1) Visa can easily available on arrival in Thailand but if you want to save more time then you can apply online as well.
2) We did Currency exchanges in India only as for visa process you need 10,000 thai baht per person /200oo thai baht or equivalent USD.
3) Carry homemade food and packed snacks item. You can have it while day tour's in thailand as you don't want to waste your money on food items which you probably wont' like and it will help to save some money :).
4) Use your bargaining skills to get better deals on tour's , check price on multiple outlets and you will get good deal for sure.
5) Buy water from stores like 7-eleven , food mart , family mart in advance as in restaurents and tour site it will be more expensive. 7-11 stores easily availble everywhere in thailand.
Most effective way to get from one city to another is to book a domestic flight to save the time. Ferry rides and cruises are also available to various islands.