If you're coming to Canada from the U.S., your transition to Canadian money will be a simple one that does not require much advance planning. Unless you have a bank that really specializes in making Canadian currency exchanges, you are probably better off not bothering to exchange any money prior to your arrival in Canada. The reason is that many banks attach flat fees for currency exchanges and also have a spread between what they sell the currency for and what they will pay to buy it back. Spreads and fees almost invariably add up to far more than ATM fees would for similarly valued transactions and there is simply no reason to pay them if you can avoid it.
ATMs are found virtually everywhere in Canada and that is your first choice for obtaining Canadian currency. (If you use a debit card, be sure to check with your financial institution or have a backup plan). Most businesses in Canada accept credit cards and you will receive your most favorable exchange rate by going that route, rather than drawing cash for expenses. The reason is that while purchases involve a "grace period" on your bill, cash advances do not. Many card issuers are now adding a percent or two for foreign currency transactions. It's not a good thing, but nothing to get overly concerned about either. The exchange rate you see in the newspaper each day involves large, interbank transactions. Your bill will be converted at close to that rate. All things considered, the extra add-ons will still leave you far ahead of the typical retail-level currency booth transaction.
While many businesses in Canada will also accept U.S. money, it's generally not a good idea for you because the exchange rate is often not the best. Travelers checks? While you can certainly use them if that is your practice; they can present a hassle to convert to cash and they may also involve a flat fee to transact. On the occasions I've used them, I've purchased them in the currency of the country where I'm planning to spend them. This enables the checks to be transacted directly for purchases (where they are accepted), rather than requiring a stop at a bank.
Finally, the lowest denomination of paper money in Canada is $5, since coins are minted for $1 (the Loonie) and $2 (the Toonie) amounts. In any foreign country, you should endeavor to use up your coins before leaving; often only paper currency can be converted back into U.S. dollars.
Save your receipts for lodging and any retail purchases that total more than $50. You can get a refund on some or all the General Sales Tax (GST), Provincial Sales Tax (PST) or Harmonized Sales Tax (HST). The amount of the tax and the portion to be refunded varies by province. You can claim the refund by filling out a form that you can find online at:
http://www.rc.gc.ca/visitors/ -- fill out the form and mail it in with ORIGINAL receipts and your boarding pass showing your flight leaving Canada (proof of export is required to claim the refund). If you are leaving by car, you can take care of your refund at the duty-free store at the border. Do NOT use commercial services, which often display brochures or run ads in visitor guides. The fees are outrageous and the claim form is very easy to complete and submit.
While you should always be careful with money and alert to unsafe situations, no special safeguards beyond what you would normally take while traveling in the U.S. are required in Canada. It's as "safe" a place as there is.