The Avengers have decided that they needed a vacation so that they can bond better as a group and so that they may relax. Thor creates the idea that they should visit Asgard for a week, and it's one hell of an experience. The culture is rich, the technology is fascinating, and Odin makes sure you live like Kings and Queens in Asgard.

For my scenario, Viper is required to come along because in Steve's words, "Vacation is good for rehabilitation". In other words, after her capture, they are trying to force her to be a good guy.