I celebrated my 25th birthday with my girlfriends in Kaliningrad where we rent a room on board a local museum ship. Spending a night in a cabin of a real ship was such a great experience that I decided to repeat it in Berlin. The Eastern Comfort hostel boat seemed to be a perfect choice. The cabins even have an en-suite bathroom with a shower and the deck is an ideal place to enjoy sunsets over the Spree and the Oberbaumbruecke bridge.
If you've almost run out of money, you can sleep right on the deck in the open air. Please note that you can either rent linen and towels on site for €5 or bring them with you. By the way, on Wednesdays the lounge area hosts language evenings - practice your skills with native speakers while listening to live music.