In addition to the Cash Wheel, there was later the Double Wheel, which was mostly merchandise with a few cash spaces ranging from a salami to a car. The contestant spun it the first time and then decided if they wanted to take the prize or spin it again and try to get something better.
The Envelope was usually done about once a month--and it was in every game until it was won. It was also set apart by the fact that every time it appeared on the board a bell rang and the camera zoomed in on the space (and zoomed right back when they cut back to the contestant calling the next number).
When it was won, Hugh or Bob would hand the contestant the envelope and ask them to open it. The contestant would do it, pull out the card and read out loud what was on it, which always led to some sort of special presentation, often with a "Treasure Hunt"-style misdirection to make the winner think that s/he was getting a gag prize before the good news was revealed (without, of course, the extremes of misdirection that Geoff would do on "Treasure Hunt"). Sometimes, it was just a series of prizes in a Showcase-type presentation. Outside of the international games, the Envelope spots were often where model Paola Diva appeared--and when a grand piano was an Envelope prize, we got a rare appearance of Milt Kaye on camera, in a tuxedo playing a little Rachmaninoff.