It's HCl, not HCI (HCl = Hydrochloride). Diphenhydramine HCl is the active ingredient in Benadryl, generic equivalents and some sleeping pills. If you take enough (300-500mg, iirc), you will trip. It's not usually considered all that pleasant. The main form hallucinations tend to take is completely lifelike people and objects, usually commonplace things you might expect to see normally, like friends and family or mundane objects. The hallucinated people often speak to you, and they tend to seem completely real. This can be quite unnerving. I never got full-blown hallucinations like that when I experimented with it, but it's kinda trippy and not too unpleasant at doses of around 300mg, at least for me.

if you want a weird, legal trip, I'd advise salvia over diphenhydramine. It's more like other trips (although markedly different; thus the "weird"), whereas diphenhydramine is more like the deleriants (which are not known for their recreational value).