Nitrous is always extremely intense. However, I try not to do it anymore since I always end up chasing the high and I can sit there doing them for hours.
Using some type of 'Rush' (poppers, butyl nitrite, etc) is also extremely intense while tripping. Although short-lived, within seconds of a hit, the trip is intensifed beyond belief and the euphoria is incredible.

It's great combined with nitrous as well. Take a hit of Rush, and then immediately follow it with one or two bulbs of nitrous.
Quality lab-grade ether is also a mind bender while tripping. Nothing like a good ether binge. I'm starting to sound like a huffer, so I'll stop with the inhalent talk now. %) I've also used several medical-grade (inahaled) anesthetics in the past with incredible effect while on a psychedelic.
I don't use inhalents often, but a little bit of any of the above is a great combination with almost any psychedelic. I don't think the ones above are really that damaging to one's health either if used in moderation.
Anyway as far as psychedelic combinations go, I have used the following to great effect:
1) 4-HO-MiPT/4-AcO-MiPT - Only done this combo once. I had finished empyting out a baggie of each chem while weighing out a few doses of each for future use. Prior to discarding the baggies, I tore them open and licked each of them clean. I wasn't planning on tripping that night and was just expecting mild mood enhancement if anything. I got much more than I expected.

During the peak, a few hits of nitrous brought me to a +4. I plan on trying this combo again at some point in the future. These two chems synergize very well with each other, so I will probably start low and work my way up.
2) DPT/4-HO-MiPT - Have only done this combo once too, but it was simply indecribable. I also plan on repeating this at some point in the future.
3) Peruvian Torch in combination with several of the 2C's - Dried outer skin 'chips' of T. peruvianus that were extremely potent were ground up and loaded into capsules. Low doses of this used to 'accent' 2ct2, 2cc and 2cd were incredible. What would normally be a threshold dose of the cactus by itself were very synergistic and did a great job with the three 2c's mentioned. I am interested in repeating it in the future as well as trying it with 2ce, 2ci, 2ct4 and 2cp at some point. %) I also think 2ct21 would be very nice to combine with mescaline.