Bearing in mind that I don't condone or recommend breaking the law, and I quit doing substances stronger than coffee and cigarettes well over a decade ago (and have almost five years off the latter at this point)....
Neither Benadryl nor ketamine are really going to get you a great 'trip' experience - there's too much else going on with the effects for that. If you're determined to go on a trip without leaving your house, save your money until you can find someone who can get you a couple of hits of decent clean LSD, or some peyote or something similar, and have someone experienced with you the first time. (Please be safe with psychoactives - shit can get real weird real fast.)