Well frequented taverns of the classic sense began to pop up in eleventh and twelfth century England. There were rooms and food was almost always served, but as it still is today, the consumption of alcohol was the most popular activity. This influenced the conversation. Of course, the conversation would have varied greatly depending on where the tavern was. We can only speculate regarding what was said, but think about what you'd hear today, and it wasn't all that different. There are example throughout literature (Chaucer) that discusses lightly what would have been talked about in bars. If the inn were at a crossroads in the country, travelers would have been discussing who they were, where they were from, what the crops were like, etc. In country inns the gossip wouldn't have been about the ruling family, but they local lord. In London everything would be a little different. There were hundreds of pubs in medieval London (13th C) and each neighborhood would have it's own, so those who frequented it would know each other well, and would gossip about friends, girls. War played a much more central role in discussions, if not strictly because people were less in formed. Today we know the details of what's going on abroad (more or less) and conversation about it is a little repetitive because the details are widely reported. In Medieval England, during the Hundred Years War, there would have been a significant amount of speculation as to what really occurred at Crecy and other battles. Where the troops were and what they were up to would be a major topic of conversation in London. People in the country would have been less informed, but that only means that the conversation would be even more diversified regarding war.

It might be pretty hard to know with much confidence. Vellum was so expensive that most of what the written record from the era was "important" (not always important, but not transcripts from the local pub).

That said, books like Canterbury Tales can probably give us a reasonable idea at least of what interested (literate) people and the topics aren't all that much different than they are today. Work, sex, God, wealth, corruption, fame, infamy.

There were no sports exactly, but Barbara Tuchman tells us this tale:

"Players with hands tied behind them competed to kill a cat nailed to a post by battering it to death with their heads, at the risk of cheeks ripped open or eyes scratched out by the frantic animals claws..."

So there's some good watercooler fodder.

As for jokes, I love a book called The Facetiae because the jokes are so gross. Here's one (slightly paraphrased):

"An itinerant quack's sign depicted a Priapus [a cock, basically] divided, at certain intervals, by band-strings [quartered by lines, I imagine].

A certain Venetian came up, and enquired the meaning of those partitions. The quack, for the fun of the thing, replied that membrum suum [his own cock] was endowed with such a peculiar property, that if, with a woman, he used but the first part, he begot merchants; if the second, soldiers; up to the third, Generals; up to the fourth Popes [that is, stuck it all the way in]; his fee being proportionate to the rank and quality ordered.

The dolt [the Venetian] took his word for it, and, after a conference with his wife, brought the quack to his house and bargained for a soldier. As soon as the quack had set about the job, the husband made a pretense of withdrawing, but hid himself behind the bed; when he saw the pair hard at work manufacturing the soldier agreed upon, the husband rushed forward, and giving the quack's backside a vigorous push, so as to secure the advantages even of the fourth division.

"By God's Holy Gospel," the husband shouted, "This will be a Pope!"

You! People in mediaeval times took far fewer long journeys than we do today (indeed, peasants and serfs were not even permitted to travel without the permission of their lord), and only rulers, messengers, merchants, missionaries, sailors, soldiers etc. moved around any significant distance[1][2]. Regional dialects were more-pronounced than they are today[5], and the unfamiliarity with you by the "regulars", coupled with your unfamiliar patterns of speech, would single you out as an outsider.[4]

In order to fit in, you might pretend to be a journeyman (especially in the area now described as Germany: a tradesman who's completed his apprenticeship but whose local guild does not have a seat for him, or who seeks to learn techniques specific to a different region) or a pilgrim heading to a distant holy site.[3] Alternatively, you might try to pass as a peddler, with wares to sell, travelling from some far-off market town with your (probably hand-drawn cart).

Possibly not the answer you're looking for, but a valid point nonetheless.

Update: sorry; was writing on phone earlier. Sources/further reading:

[1] Everyday Life in Medieval Europe, Neil Grant
[2] http://www.localhistories.org/middle.html
[3] http://hyperhistory.net/apwh/essays/cot/t2w08pilgrimage.htm
[4] Have you tried reading The Canterbury Tales (Chaucer)?
[5] http://www.ling.upenn.edu/~dringe/CorpStuff/Thesis/Dialects.html