When Google employees returned to their mostly empty offices this month, they were told to relax. Office time should be “not only productive but also fun.” Explore the place a little. Don’t book back-to-back meetings.Also, don’t forget to attend the private show by Lizzo, one of the hottest pop stars in the country. If that’s not enough, the company is also planning “pop-up events” that will feature “every Googler’s favorite duo: food and swag.”But Google employees in Boulder, Colo., were still reminded of what they were giving up when the company gave them mouse pads with the image of a sad-eyed cat. Underneath the pet was a plea: “You’re not going to RTO, right?”R.T.O., for return to office, is an abbreviation born of the pandemic. …