Here's the scenario in a nutshell:
Two people with Thai passports (they have a 10 year entry/exit to USA visa). They will be in the US. They want to visit Niagara Falls. Obviously, they want to visit the Canadian side of the falls (see the Funicular and the Journey Behind the Falls).

Will they need to pre-arrange a Canadian Visa to walk across the Rainbown bridge to enter Canada?
Is there a difference in visa requirements if you walk across vs drive across the bridge?


Follow-on question:
We'll be driving from Detroit to Boston. The plan is to stop in Niagara to see the falls (hence the question above). I notice that saves about an hour and a half to cut through Canada (rather than go around Lake Erie). What would the visa situation be? It will be me (US citizen), my GF (US citizen) and my future in-laws (Thai Passport situation described above).

Do they need to pre-arrange a visa for the 4 or 5 hour road trip across Canada?

Thanks in advance.