I'm traveling in US on a tourist visa and want to also visit Canada for a day. Is there anything that will make it difficult for me to get back to the US? For example, are there laws on how soon you can return back to US as a tourist?
Be very cautious about this! It depends on whether your visa is "single entry" or "multiple entry".
From the US State Department:
Depending on your nationality, visas can be issued from a single entry (application) up to multiple/unlimited entries.
If you have a single entry visa and leave the US for Canada, you will NOT be allowed to renter the US from Candada. You'll have to return to your home country and apply for a new US visa. I know of grad students who went on a day trip to Vancouver BC from Seattle and were not allowed to re-enter the US from Canada. It was a disaster for them.