I should have swapped tips with MrTall since I've just been to Lithuania and didn't rate Vilnius in comparison to Riga!
Really like Riga, I think it is a great city. Stag dos present but not half as irritating as in your average british city.
We went for NYE two years ago, great atmosphere in the city centre, loads of people and snow. Only trouble was somewhere to eat on NYE so plan ahead for that.
I recommend a day trip to Jurmala, stunning coastline, and an evening in the Aquapark there, it is brilliant fun.
Also ice skating at the out of town Lido complex and a trip to the gigantic indoor market.
Can't recall many of the bars but really liked the Skybar on top of the Reval (I think that is the name) hotel.
Hope some of that helps.
keppoch.