The dose required to reach any effect in most people is very high, lets say from 10-20x the usual. Diphenhydramine/dimenhydrinate are sold over the counter for motion sickness and sleep induction, doses this high would probably make you extremely drowsy.
You may see things, but are not aware that they are not real. For instance in some of the experience reports people have said they began talking to people whom they thought were present (friends or relatives), the individuals would then disappear and come back. Others describe having visual distortions or hallucinations that were not pleasant.

Look into it yourself: http://www.erowid.org/pharms/diphenh...ydramine.shtml