They would have continued dropping nuclear weapons (their production schedule was around 3 bombs a month, and a third would have been ready for use about a week after Nagasaki), continued conventional bombing, and invaded as planned. This assumes, of course, that Japan would not have surrendered anyway before November 1st (which they probably would have, even without the bombs — and it is not entirely clear that the atomic bombs are what made them surrender, anyway, but this is a separate matter).

The planned invasion of Japan, Operation Downfall, was set to begin with Operation Olympic which would have stormed the beaches of Kyushu (the southernmost of the main islands of Japan) on November 1st, 1945, less than 100 days after the the atomic bombs were dropped on Hiroshima and Nagasaki.

Given the size and scale of the invasion, which would have included over 40 aircraft carriers and 20 battleships, the forces were already being marshaled in the Pacific for the invasion at this point on islands such as Okinawa. Troops in Europe had already been trickling into the Pacific in anticipation of said invasion.

As thus, had Japan not surrendered after the atomic bombings, the invasion would have proceeded as planned.

You can read more about it at Reports of General MacArthur, straight from the US Army's website