Canadian Border Security wants to make sure you are a visitor rather than a burden or planning to take a job or engage in an unlawful act. A burden would include someone not medically fit or someone who has no funds.
You will be rejected for any criminal convictions or outstanding warrants for a crime that would be considered a crime in Canada. Usually you are OK if the penalty of the crime ended 10 years ago. DUI is included. You may be rejected if you have not enough funds or have no job in the US.
Tips:
Passport.
No weapons including any spring-loaded knife, pepper spray, bat, handgun.
Have some cash (not necessarily Canadian) but must declare anything over $10,000.
No drugs. Even if legal in Canada it is not legal to cross the border and you will be arrested for smuggling. No pipes/syringes etc. Make sure prescriptions bottles have your name.
Have an itinerary showing where you are staying or print out your travel route. Have specific sites you are seeing or things you are doing in Canada
Have a specific return to US date.
Canada has some rules against certain fruits and meats entering the country so bring no food. Alcohol and tobacco can be brought but sometimes that's a hassle so don't.
Don't be intoxicated as Border Security are law officers and will arrest you.
Normally crossing the border is about 5 minutes (excluding lineups). There is a random selection for closer inspection and it is fairly random and not based on appearances. Most smugglers are well dressed and well groomed.
I live in Vancouver and cross the border a few hundred times and usually not a bad experience but periodically I get checked due to random sampling.
For details:
bsf5082: Visitors to Canada and other Temporary Residents
G.