Our Dean is constantly focused on improving our library space and services. By offering this event, he gets 90 minutes to chat with students to hear about their experiences, good and bad, with the library and other academic services. This has lead to a lot of positive feedback, as well as manageable changes that we should make. We hear about hours, availability of resources, library anxiety, and many other candid topics that impact students most directly. It's a constant reminder to keep the students and faculty at the forefront, and the "library" second.
I would highly recommend this outreach/engagement activity. The students love it, there is a relatively low cost, and it generates a lot of good will for the library.
We ordered our coffee through the campus catering service. It's a little more expensive than paying for coffee by the gallon at a retail location, but the added benefit is that they provide the tables/table cloths, coffee, tea, cups, napkins, plates, and cutlery. They also manage the set-up and take-down of the event. We ordered our donuts from Dunkin Donuts - our past events dictated how many donuts to order. This data log helps us make a good decision about how many to order for future events. I would also use your weekly floor/gate count to estimate how many people may be in and out of your library during that time frame. If you have a local franchise or other establishment in your neighborhood, ask about any discounts they may offer for the school, or non-profits. We've had fairly good luck getting small discounts from a variety of vendors in the past.