Let's talk weed adjuncts. I have two that I've found useful:
Powdered Mulungu bark (Elythrina Mulungu): Marketed as an anxiolytic, but I didn't find it effective for that. Appears to be a nicotinic acetylcholine receptor antagonist. I find in combination with weed it enhances the psychedelic effects, but probably reduces the euphoria. Consumed by making a tea out of it.
Blue Lily/Blue Lotus (Nymphaea caerulea): Only got this the other day, big fan. It's consumed by smoking, which is convenient for potheads. It produces a mild but very pleasant clear-headed high, a feeling of connection and some subtle entheogenic/entactogenic effects, heading in the direction of MDMA but not so intense of course. Some sedation as well, which persists longer than the high. I found making a tea ineffective, I haven't tried soaking it in alcohol, which is another recommended method. Unlike Mulungu, I'm not sure it really "synergizes" with weed exactly (probably moreso alcohol) but it's nice in combination all the same. Works fine on its own too if you want a more clear-headed high than weed.
To the best of my knowledge, both of the above are legal and not dangerous, but do your own research of course.