I go by ferry and train from Ærøskøbing to Kalmar on Friday, May 18. After two nights in Kalmar, I go by train to Stockholm on Sunday the 20th.
I've been trying to figure out whether spending part of my Kalmar time on Öland is worthwhile. I posted on Rick Steves' Travelers Helpline and on Lonely Planet's Thorn Tree forum, and the responses I got lead me to think I should definitely go.
I will arrive in Kalmar fairly late on Friday evening. The train from Svendborg to Kalmar is about 6 hours or longer, and that's after a 75-minute ferry ride from Ærøskøbing. I can take a ferry at 11:45, and then there's a train at 14:12, arriving in Kalmar at 20:50. Or I can take a ferry at 14:45 and catch a train at 16:12, which is a quick transfer, arriving in Kalmar at 22:00. Either way, I don't plan on having tourist time in Kalmar on Friday night.
On Sunday, the train to Stockholm is about 5 hours. The B&B where I'm staying in Stockholm has check-in until 21:00, so the latest train I can take leaves at 15:04, arriving at 19:50. That still gives me a good portion of the day on Sunday in Kalmar. So if I wanted to, I could do a full day on Saturday on Öland and still have plenty of time on Sunday to spend in Kalmar.
If I do that, I could rent a car, and that way explore more of the island, or I could take a bus. Having a car would allow me to wander as much as I want around the island, which is 137 km long and 16 km wide at its widest point. But it's pricey (over $100 and possibly closer to $200 for a one-day rental), and as I've learned from other driving vacations I've taken, it's tedious to be a solo driver sightseer. It's one thing to drive to destinations, but I suspect touring Öland would be like driving the Cabot Trail, which was a wonderful but long day of pulling over to enjoy views and jumping back in the car.
I found a blog post from someone who did this in 2008. He said he logged 200 miles and it was a very long day. I wouldn't have to do the entire island, but still, I am leaning toward just hopping on a bus and visiting Borgholm. The bus ride is a little under an hour each way. I could potentially rent a bike too.
UPDATE (March 14, 2012)
I got some more advice that led me to go ahead and reserve a car for Saturday May 19. I don't have to pay in advance (didn't even need to provide a credit card), so if I need to cancel, there won't be a problem.