One thing that hasn't shown up during the excavations and renovations, that everyone expects will, is dead bodies. That's not to say a lot of human remains haven't been found -- 148 cases of bodies or skeletal remains have been uncovered in the desert or in other "unusual" locations (i.e., not businesses or homes) in Clark County since the 1950s, for which you can see details on the interactive map compiled by Channel 8 News -- but no hotel renovation/rebuild has turned up any bones, or not that the powers that be have chosen to make public, at least.
Time capsules, of which a few have been buried around the valley over the years (see "QoD" 3/10/2008) have had mixed fortunes at being rediscovered. In the summer of 1969, ground was broken on the 14-story Centurion Tower at Caesars Palace and a time capsule was allegedly first buried on the site, but was apparently stolen before the first concrete was laid a week later.
Prior to this, back in December 1953 a time capsule was buried at the site of the former Sands (where Venetian now stands) to commemorate the property's first anniversary. The 12-foot-long rocket-shaped container, which contained all kinds of interesting memorabilia*, was supposed to remain interred for a century, but was apparently discovered a mere decade later in the landfill near the North Las Vegas pig farm, having been inadvertently dug up and dumped during some construction work.
Three days before Steve Wynn purchased the Desert Inn, in April 2000, another time capsule was buried to commemorate the venerable hotel's 50th birthday. Wynn closed the property a few months later and imploded it to build his eponymous new resort. We've found no word of the fate of that capsule, which may well still be somewhere in the foundations of what is now Wynn Las Vegas.
As far as "finds" are concerned, the only other one we've heard about was the story you referenced, regarding what turned out to the "Sports" section of a 1979 edition of the Milwaukee Journal, showing up inside one of the dragon sculptures during the Imperial Palace's transformation into The Quad. You can read more about this random find on CET's official Pulse of Vegas blog.
As to items not found, but rather rescued or salvaged from now-defunct properties, there are several interesting examples, some of which we covered in a prior "QoD" (see the 12/8/12 in the Archives) -- a list to which we have a recently discovered an interesting addition -- but that's another story for another day...
