First off a camera, obviously!
DSLR camera and tripod, and because it's remote and far off in space, the pics will be worth a fortune if I get back to Earth.
Even if I don't then at least I can catalog the sky, terrain, local flora and fauna, whatever happens to be around this "far off" location.
(Capturing a whole series of sky images may also help me find my way back to earth or at least work out roughly which direction I've travelled in if I can recognise familiar star patterns, and I'm not too far away from our little neighbourhood of the galaxy.)
Secondly I'd like to have a laptop with massive amounts of storage for storing pictures from the camera, all my favourite music files preloaded, a few favourite games, etc. I could keep notes / maintain a journal of all that befalls me there too.
Thirdly ...... hmm, now it gets tough. So many things to choose....
I'd hate to leave pizza's behind me forever - but then if I HAD a pizza there how could I eat it, knowing it would BE the last one I ever eat....
I like a beer occasionally, same goes for a cold can of coke, and a Bundy rum, but I can live without all of those....
I love a good read, but in this situation I'd be living an adventure greater than any I could read about in any book....
I think to maintain sanity one needs companionship - and I think I have to vote with Alex, I'd take my 2 faithful dogs along!
(Joy - if you somehow read this - don't be mad I didnt "choose" you, you can make your own list of 3 things and bring them along with us.)