The kappa and delta receptors play no real part in the effects you mentioned (euphoria, addictiveness, pain-killing, respiratory depression). Those are all effects are generally attributed to the µ-Opioid receptor. At the cellular level, they all activate G-proteins (35S-GTP gamma-S binding), activate inwardly rectifying K+ channels, inhibit calcium Ca2+ currents, and inhibit adenylyl cyclase, but the mu obviously posses many other properties that have yet to be fully extrapolated. We stil don't really understand any of the receptors. It is only in the last few years that they have been able to solve the exact structure of the various receptors by x-ray crystallography and protein NMR. Having the full structural conformation of the receptors allows us to better understand ligand docking, but we're still clueless about how the subjective effects are actually elicited. Apperently, when the ligand finds it's way into the binding pocket, and arranges itself into it's binding conformation by coordinating with the active sites on the amino acid residues in protein pocket, and the protein actually changes shape, rearranges to it's "agonized" conformation until the ligand detaches. This, of course is what elicits the subjective effects.
For our purposes, there are a select few receptors that we are looking to target, namely the µ-Opioid, 5-HT2A, CB1 Cannabinoid, Etc. This is, of course, not meant to imply that the other receptor subtypes are without merit, just that for what we are looking for (= fun compounds), they're just not the place to be.
I just came across an interesting compound. N-(4-Bromobenzyl)-5-Methoxytryptamine. Apperently it is a very potent and selective ligand for the 5-HT2A receptors (Ki= 0.1nM) It was reported by Glennon et al: "Influence of amine substituents on 5-HT2A versus 5-HT2C binding of phenylalkyl- and indolylalklamines." J. Med. Chem. 1994 37 1929.
This could very easily be made from 5-Methoxytryptamine (which could be made from melatonin) by reacting with 4-Bromobenzylchloride in CH2Cl2 catalysed by DMAP or Hünig's Base (DIPEA).