When it comes to tiny, San Marino (its official title, The Most Serene Republic of San Marino, is more of an endorsement than a title) is a colossus of the miniaturised: in its entirety it's not much bigger than two or three suburbs strung together. But it is the world's oldest surviving republic.
The best period to visit is April to June. The weather is sunny without being stifling, the countryside bursts with spring flowers, and the flood of summer tourism has yet to crash over the peninsula. July and August - in which prices soar, tempers flare and the country broils - are best avoided.
For something a little extra, you might want to coincide your visit with the 'Medieval Days with Antique Crossbow Competition' - what's a castle and three fortresses without a crossbow or two. Failing that, 3 September is when the San Marinese brush off the crossbows to celebrate National Independence Day.