Generally, any pre-packaged cold foods that have a kosher symbol on them are fine. The 3 widely used kosher product symbols are "OU", "OK" and "Chaf K". (I won't delve into the various opinions of who relies on which symbols, as that's too complex to list, here.) But you will find one of these symbols on a wide array of products from dairy items to pre-packaged cookies and cakes, coffees, teas, etc.
If the items are cold, ready-to-eat and you can supply plastics / paperware, I think you will have covered a wide range of possibilities. Heating items in a mixed-used microwave becomes a problem.
With coffee and tea, etc. it also depends if you are using a plain hot water dispensr or the machine dispenses non-kosher coffee or other drinks as well.
Pre-packaged sandwiches, would require a kosher certification on the packaging, and even then, can't be an item that would be heated (in a "public" microwave), as stated above.
Fruits and veggies have to be whole - non-cut.
You could also offer kosher packaged sliced bread. It would help to leave the bread in its original packaging so those that need to can verify its kosher status. Toasting the bread in the public toaster would be problematic.