We are thinking of going from Argetnina into Brazil for a day. What does it involve? Is a VISA required? Is crossing the border a long and/or difficult process?

If you're a US or Canadian citizen you need a Brazilian visa. I know people who have crossed into Brazil from Argentina without a visa and didn't have problems. A few years ago, I crossed into Brazil from Argentina at Iguazu and the Brazilian border guard did check my passport. I had a visa, so no problems.

I'm a Canadian who travelled to Brazil from Guyana last year. Not only was a visa required but a yellow fever shot. We had arranged for the visas beforehand. The yellow fever shot we were able to get at the border. The border however does close from noon until 2:00 p.m. so travel arrangements had to made around those hours.

One day... better spend it in Argentina..you will see nothing in such short period of time...Brasil is huge and lots to see and do..save it for your next trip.
And yes you need a visa...

The visa will cost $100 per person and is good for 5 years as long as you enter Brasil within 90 days of the day you get the visa otherwise it is not valid anymore.

And do not forget to get the stamp from the immigration officer when entering Brasil, such stamp activate your visa for the next 5 years, otherwise you will not able to use visa again... tricky but they do it.

Who do you contact for a Visa and how long do these take?

Just wondering as well what happens if you go to Brazil without a Visa? Would they send you back?

NO AIRLINE SHOULD LET YOU BOARD PLANE WITHOUT VISA...AND IF THEY DO, AIRLINE IS FINED.
You will not be allowed into the country..send back at your expense. If officer is moody, you could be deteianed for interrogation.

In February 2005, we hired an taxi to go from Argentina Iguazu Falls to the Brazil side for the day and did not need a visa. At the border crossing, the cab driver showed our passports to the control officers and they just let us through. It took five minutes. Several others from our hotel did this and no one had a problem.