As others have said, if you've booked already 94 days is not ideal. One option, if you're over 50 is the o-a visa if you really want to remain in thailand the whole time and don't want to bother with immigration visits. The best alternative would probably be 60 day tourist, a few days in Vietnam or Cambodia and then re-enter again on visa exempt
i'm not sure what the costs are in Canada but you'd probably be looking at about $200 for the o-a visa and $40 for the tourist visa. O-A also requires a medical and a police check which will cost you. All for essentially just 3 days. I'd personally think the money would be better spent on a few day at Angkor Watt or similar
not really as you can use it at places that take visa/MC with no additional fees and you'll have locked in the exchange rate thats good for you. Some banks charge foreign transaction fees when using their cards in any circumstances