Dont do robitussin, its not worth the nausea and stomach pains just to be confused and have tunnel vision for three hours.
i would tell ya to do shrooms. if you do, try eating just a half a 1/8 for the first time shrooms are a shit ton of fun (i didn't mean that to rhyme,btw)
Work out the day you're going to trip, if you can. Get those endorphins flowing and feeling great! I was almost contemplating not doing shrooms because I was having a bad day, but I worked out, started feeling great, and had a mindblowing experience.