I have used this service a few times. There is a restaurant car attached from the Polish border (Rzepin) to Warsaw so it's available for breakfast in the direction of Warsaw and dinner in the direction of Amsterdam.
To be honest, there isn't really anywhere to go if you don't want to lie down as the whole carriage is full of compartments. There is a 'seated sleeper' carriage which is basically just First Class seats but I don't know if you will have access to it from the couchette carriages. It is not uncommon for people to leave their compartments and just stand in the corridor for a while though.
All the couchettes are Deutsche Bahn couchettes and are the same as those found on CityNightLine sleeper trains.
Berths 1 and 2 are the bottom two, berths 3 and 4 are the middle two and berths 5 and 6 are the top two. For example, if you have berth 56 then you are in 'compartment 5' (though it is labelled with numbers 51-56) and in one of the top two berths.
You may find this page helpful as it lists the formation of the train (search for EN446/447) and so you can try and find out in advance what part of the train your carriage is in as the train is pretty long.
http://www.grahnert.homepage.t-online.de/r111-400.pdf
Finally, this page contains lots of useful info about the train
http://www.seat61.com/Poland.htm
I hope this helps. A lot of the info is relevant to sleeper trains in general, so I hope it wasn't just repeating stuff you already knew!