The biggest thing about San Marino is its history. San Marino is the world's oldest surviving republic, even if it is only half the size of Liechtenstein. The third smallest state in the world behind Holy See and Monaco, San Marino is an autonomous continuation of Italian life and culture. Because it is its own sovereign state however, it is packed with tourists on their trips around Italy.
What to do
The main city is the most beautiful part of the country, the rest of San Marino is a more industrial, tiny version of the central city, also called San Marino, located on top of a mountain. Tour the three towers depicted on the flag, or just walk around the streets of San Marino. You'll be treated to views and welcome surprises as you walk around the tiny, cramped walking streets.
You can't fly to San Marino because there aren't any airports, so you'll have to find a flight to Italy before you can tour San Marino. There also aren't any train stations or bus lines running into San Marino so rent a car if you're sure you want to come into this tiny tourist attraction of a country. The border with Italy is completely open, so there's no need for a border control or any sort of visa or trip permit.
Where to stay
There are hotels in San Marino but the selection is limited because of the city's small size. You'll have better luck going to one of the surrounding villages or towns in Italy and looking up a hostel or hotel there. Check out the Italy travel logue for more advice on visiting San Marino.