First thing, I seriously doubt you will be able to "do it once". DXM is one of the most addictive hallucinogens ive ever experienced, yes, the trip can be THAT good.
Second, you will not be able to act sober, at all. Out of all the crazy drugs ive done, cough syrup has dismantled me from reality the most. On DXM you stumble, can barely speak, and if you can speak you say nothing but nonsense.
Third, if you do smoke weed, keep in mind you will probably only require a very small amount. Weed amazingly enhances the trip, but you will not need much at all.
And I don't mean to be the bearer of bad news, but cough syrup is a drug I don't recommend you try. I am very headstrong, but I fell in love with that shit so much that I was doing every weekend, until I had a seizure. Since then, I had very long term tinnitus (ear ringing) that occured when i smoked pot or sat still, and it took over a year to go away and to this day I will experience it to a minor extent