Sounds like you'll be in town during the prettiest time of the year; I'd recommend riding Millcreek Canyon if you have the chance

And, I'd offer to loan you one of my rides (I'm in northern Utah for the next couple of months), but they're much too small; for you they'd be sorta like riding a clown-bike!

The above-mentioned advice, re: KSL.com is good; the locals use it (rather than CL) for bike listings, if you're looking to buy a disposable bike for a few days. That said, they do have a more-or-less functional bike rental system in downtown SLC, and Guthrie's is the place to start for LBSs. Salt Lake Bike Collective has a good word-of-mouth rep, but never having been there, I personally can't vouch for them.

Wasatch Touring & Contender are both good, but last time I dropped by either one (admittedly this was a decade ago, so things might have changed since then), they were the kind of shops were you wouldn't get the time of day unless you were riding the TDF or the SOC.