WordNet (r) 3.0 (2006):

sure thing
    n 1: something that is certain; "his victory is a certainty"
         [syn: certainty, sure thing, foregone conclusion]
         [ant: precariousness, uncertainness, uncertainty]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "sure thing":
   OK, Roger, absolutely, all right, alright, alrighty, amen,
   as you say, assuredly, aye, best bet, by all means, certainly,
   certainty, cinch, da, dead-sure thing, exactly, favorable prospect,
   fine, good, good chance, good enough, good opportunity,
   good possibility, hear, indeed, indeedy, ja, just so,
   lead-pipe cinch, likelihood, main chance, mais oui, most assuredly,
   natural, naturally, naturellement, odds-on, odds-on chance,
   of course, okay, open-and-shut case, oui, positively, possibility,
   precisely, probability, quite, rather, really, right, righto,
   shoo-in, sporting chance, sure, sure bet, sure card, sure success,
   surely, to be sure, truly, very well, well and good,
   well-grounded hope, why yes, winner, yea, yeah, yep, yes,
   yes indeed, yes indeedy, yes sir, yes sirree