Diphenhydramine is not a psychedelic by any means. It is a delierient, and the hallucinations you may achieve are nowhere close to being enjoyable, in my experience. A common thing to see is spiders/bugs crawling over everything. Sucks when you are an arachniphobe. Before I knew much about diphen I took a large dose to try and come down from doing a lot of cocaine, and had the worst hallucinations of bugs and shadow people and shit. Horrible.
I take diphen to sleep on a regular basis, and take up to 200mg on occasion to sleep. To achieve a "trip" on it, you would probably need atleast 400mg in my experience. You may get minor hallucinations on it at lower doses, but a high dose is needed for full effect.
Many times the hallucinations are very real, and it is hard ti differentiate between reality and you hallucinations. it is common to have conversations with people who are actually not there, although theyappear to be.
I would compare it to amphetamine psychosis.. So if that's your idea of fun, then do it up.