It depends a bit on what you are looking for. Öland is a great place to visit, but there are other options as well if you have a free day in Kalmar.
There are a couple of towns and villages on the Island, the largest of them being Borgholm. And just outside Borgholm are the ruins of an old fortress that are a popular place to visit, https://en.wikipedia.org/wiki/Borgholm_Castle And just south of the ruin is Solliden, the summer residence of the royal family which is usually open to visitors in the summer. Another sight is Eketorp, https://www.eketorpsborg.se/eketorp-fort/ and the light houses on the northern and southern ends of the island. But for many, the nature is the main attraction on the island.
Regarding restaurants, there are a number of them serving the local speciality Kroppkakor which you should try, one option is Aronstorp just outside Färjestaden, https://kroppkakor.se. They are open daily until 8:30-18:00.