WordNet (r) 3.0 (2006):

cook up
    v 1: prepare or cook by mixing ingredients; "concoct a strange
         mixture" [syn: concoct, cook up]
    2: make up something artificial or untrue [syn: fabricate,
       manufacture, cook up, make up, invent]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "cook up":
   ad-lib, angle, beget, breed, brew, bring forth, bring into being,
   cabal, call into being, coin, collude, complot, conceive, concoct,
   connive, conspire, contrive, counterfeit, countermine, counterplot,
   dash off, design, develop, devise, discover, do offhand, dream up,
   engender, engineer, evolve, extemporize, fabricate, fake,
   fantasize, finagle, finesse, fix, forge, formulate, frame,
   frame up, fudge, generate, give being to, give rise to, hatch,
   hatch a plot, hatch up, hoke up, improvisate, improvise, intrigue,
   invent, jury-rig, knock off, lash up, lay a plot, machinate,
   make do with, make up, maneuver, manufacture, mature, mint,
   operate, originate, pack, pack the deal, plan, play by ear, plot,
   prearrange, preconcert, precontrive, predesign, premeditate,
   preorder, procreate, rig, scheme, scrap the plan, set up, sew up,
   spawn, stack the cards, strike off, strike out, think out,
   think up, throw off, throw together, toss off, toss out, trump up,
   vamp, vamp up, wangle, whip up, whomp up, wing it