Mid-twenties, trendy(ish) guy staying over in Paris for one night on a business trip. Prefer a cool, quiet bar to loud clubs - hopefully somewhere that won't break the bank either!
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A few years ago I got off a train and it was pouring and a friend and I ran into the closest bar we could find near our hotel, which happened to be Le Vin Au Vert. It was quiet, warm and unpretentious, with an entire wall full of wines and a lot of happy young groups of friends. I haven't been in a few years, so please double check this, but if it's still around, I highly recommend.
While I don't know the bar scene in Paris well, I think I know where to look: http://www.timeout.com/paris/en/bars-pubs This link from Time Out Paris lists what they say are the 100 best bars in Paris including best bars by area, cheap bars, dance bars, after hour places, pubs and such. There's also a link to a map that pinpoints the bars.
When I arrive in Paris for a visit, the first purchase I make is a travel card of some sort for the Metro. The second purchase I make is Pariscop, a list of what's on each week in Paris.
When I hit Heathrow, I pick up an Oyster Card and a Time Out London to read on the train to Victoria Station.
Enjoy your trip,