One of the surprising reasons? Guests are spending more time exploring the hotel. "Millennials hang out in lobbies more than they hang around their guest rooms," said Bjorn Hanson, divisional dean and clinical professor for New York University's Preston Robert Tisch Center for Hospitality, Tourism and Sports Management. "Frankly, they feel even more comfortable spending time there if they're making a purchase."
In response, hotels have beefed up services—creating unique bars, seasonal restaurant menus and all-hours cafes that are, in some cases, replacing traditional room service. Done well, those amenities can also attract locals for a further revenue boost, Hanson said.
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Part of the spending increase stems also from a shift in the mix of travelers. "The strongest gains we're seeing are in commercial business travelers, high-income travelers and groups," said Robert Mandelbaum, a director of research at PKF. "Those are the types of people who take advantage of other amenities, who are still staying in full service hotels."
Business travelers in particular are more likely to take advantage of expense accounts for room service and on-site meals, as well as hotel services for work activities, he said. Per PKF's figures, hotel audio/visual equipment rental rose 7.9 percent last year, and banquet services, 4.1 percent.
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Of course, not all of the spending increases add to travelers' fun: Hotels raked in 7 percent more on parking fees last year, too, and 5.8 percent more in miscellaneous service charges.
Travelers who want to splurge shouldn't necessarily book packages bundling in spa services, meals or other extras, said Kelsey Blodget, manager of editorial operations for review site Oyster.com. Price them out first. "Our breakdowns show, it's often cheaper to pay for the elements individually," she said. "But the guests like the convenience of having the hotel take care of everything."
—By CNBC's Kelli Grant