I'm so glad my friends were willing and able to make this trip happen - I was much too distracted with sick dogs and moving preparations to do any trip planning myself. Overall I had a fantastic time and would totally recommend a visit - if you have a car and some serious time on your hands. It's very difficult to reach San Marino by public transportation and even with a car it took 5-6 hours of (easy highway) driving from Rome. Not to put you off or anything, just be aware this trip is a real time commitment.
So, back to the fun stuff! San Marino is a tiny country - only 23.6 square miles and feels like more like a hill town when you visit. The main thing to see in San Marino is a series of three towers/castles on the highest point of the hill. Two can be toured and there's a nice hiking path that connects all three. There are also a surprising number of museums given the country's size. We happened to visit during some sort of comic book festival which added a weird vibe (museum, historic tower, basilica, Spider-Man...) but also gave us lots of great people watching opportunities.
If you go remember to bring your passport - in the center of town at the info point you can have it stamped for 5 euros - and speaking of Euros, you can ask for change in special coins minted just for San Marino. They make a nice keepsake. Other than sightseeing and collecting souvenirs, I would highly recommend devoting some serious time to eating. We ate at La Terrazza and it was amazing. (Try the local wine.)
Random fact: San Marino has the most relaxed gun laws in Europe - hence a gun store on seemingly every corner. Considering that there were no vehicle checks or customs when we left I wonder how many of those guns are just floating around the EU?
So there you go! I did make it to another country after all! Here's the proof: