I went to Warsaw on my stag-do last year in march. got some v cheap flights frid-sunday about £80-100 cheapo N class on BA. stayed at the Hyatt Regency just outside the main centre only about £60 per room per night and 5 star luxury!
taxi into the centre is dirt cheap (provided you get the licenced ones and not the mafia run ones).
adult entertainment much more risque than england's equivalent and local beer is cheap too. the Hyatt has a casino that closes at 10am! went to an excellent night club after attaching ourselves to some locals got back to the hotel at 5am and promptly lost some 'slotties' (or whatever they are called) in the casino until 9am just in time for breakfast - perfect stag weekend....

Plus going abroad, you mate's are usually less inclined to tie you to a lamppost / other horrible pranks than say newcastle...