My biggest writing bonus here so far has probably been a long visit to the British Museum. Museums are fantastic sources for people who write in settings other than the here-and-now. Quite possibly, they are even better than graveyards.
Of course, one of the reasons the British Museum was great was to get a look at entirely different cultures, and just being here supplies some of that, too. Britain gives fantastic glimpses of the past, too. Towns contain random buildings that are older than the United States is as a country - not to mention the medieval walls, Roman baths, and so on. All excellent for someone who writes medieval-based fantasy.
Someone recently asked me why so many fantasy writers set their stories in medievalesque worlds. The simple answer, as far as I know, is as follows:
A. King Arthur mythos
Truly, fantasy writers tend to be fantasy readers, and a cycle has evolved in which those who will go on to write fantasy are heavily influenced by fantasy that takes place in medievalesque settings. There are advantages to writing this kind of fantasy. One of these, admittedly, seems to stem from the aforementioned cycle: Because it's what they're used to, it's easy for readers to enter a world that isn't Earth, but is like Earth five hundred to nine hundred years ago, or thereabouts. It's easy to come up with powerful magic, because magic can be very strong just by doing things that are easily accomplished with modern technology, but cannot be done by nonmagical means in the medieval-type setting. Also, I think medieval times lend themselves to fantasy because so many people really believed in magic back then.
For me personally, it has largely to do with the toys. Assuming you're writing human characters, people are people are people - human motivations and emotions have not fundamentally changed in the past few thousand years. Given that, why not choose a backdrop you like? I enjoy exploring the implications of a world with magic, several intelligent species, and no religions. I also like castles, horses, swords, precious metal coins, and having an excuse to call a shirt a "tunic." :P
That said, I do see great opportunities in terms of non-medieval fantasy. Dragons Over London was set in the modern world, and I'll probably set things there again. At the moment, though, I have a lot to say and do with my fantasy world.