Officially, to reenter the US overland the standard requirement is one of these: (a) Passport, (b) "Passport Card" (a sort of passport-lite, works only at land or sea crossings), or (c) HomelandSec-compliant "Enhanced ID", which only a handful of state DMVs issue yet (it includes an embeded chip that can be read by the border officers' scanners).
HOWEVER when doing leisure and business in the Border zone, if you do not go beyond the zone itself or remain past a certain amount of time, you may be allowed reentry by just having valid officialy-issued ID and proof of citizenship. The CBP staff at that crossing are quite accustomed to Americans crossing for the sole purpose of partying in Tijuana anyway, and they probably also get to see them at their worst after a night of fun, so the degree of hassle you get over how good are your papers will likely depend on how well you fit some profile or another. Showing your VIP Club cards will NOT be considered a funny gag.
All reports are if you stick to the tourist zone in TJ one need not fear much more than a vulgar street rip-off, but of course, everyone should be streetwise as you would in any funky neighborhood. Get the contact info for the US Consulate in Tijuana and keep it somewhere safe in case you need it.