tonics could be like little potion bottles that you can drink to gain really cool buffs (for example, unlimited cotton) for a short period, then a debuff afterward (example, take away all cotton). you just take the top off, drink it like you would hot cocoa, and then you get the buff, and once the buff wears off, you get the debuff for a short period afterward.
Cauldron
the cauldron is a pillow that you can put multiple tonics into, and then click a button or whatever for it to spit out a combined tonic, which is a tonic with the buffs of all the tonics you put in, and afterward, all the debuffs.
how to get
I think it could be like, on one of the techwebs (likely the forest one) and you buy all the tonics for 100-300, and then the cauldron for 800. OR, if you wanna go all out, there could be an alchemy store (like the RC vehicle store) somewhere that you buy them all from.
I think this would be a really cool idea that would make yeeps a lot more exciting and fun.
some tonic ideas
cotton boost: you get unlimited cotton for a little while, but afterward you're left at 0 cotton.
speed boost: you get the radiation speed buff, but then the coal's slow debuff.
critical hit: instead of a passive effect, this one gives you a critical hit (3x damage) for a single hit, but drinking it makes you take critical damage next time you get hit.
blast resistance: you won't be knocked back/hurt by explosions for a while, but afterward you blow up.
growing: become really big, able to pick other players up and throw them around (that might be hard to code in, so if you can't you could just make it a damage boost or something) but afterward you shrink back down to regular size and your cotton caps off at 2500 for a while.
And you could add any other ideas you have for tonics. :D