At different ceremonies you may encounter people who surround themselves with stuff: crystals, little vials of different healing potions, favorite objects, shoe options, etc. I like to keep things simple. At a shamanic centre, a mattress will normally be provided and some pillows. Let's assume here, though, that you have to bring your own stuff. I prefer an inflatable camping mattress to lie on. Note that this is not the thick kind you inflate with a bicycle pump, but rather the thin kind hikers use, that can be rolled up tight. Anything more than that and I'm rolling around too much; anything less and I become sore from the hard floor. I divide my time during ceremony either lying back propped up on pillows or sitting up fairly straight, in a meditation pose. (I rarely lie down flat except for stretching out my back — the visions just become to overwhelming.) As I can't easily sit with my legs crossed for a long time, I find it very useful to bring along a yoga chair (there are versions used by hikers) that's basically a legless item made from two pieces of nylon-encased foam, held together with straps. I bring a sleeping bag rather than blankets, and usually keep this unzipped like a duvet. I also bring a water bottle. Note that I only sip the water as needed.
Also I strongly recommend to check twice that you have a bucket for vomit and papers to clean your mouth and face.
Practically in a good centre you will be packed with all this stuff except a few things like yoga pillow and mattress.