I've stayed at the DoubleTree Suites many times. I've never done this upgrade, but I have asked about it twice. It is basically a tray that comes with various sweet and salty snacks. On multi-night stays, the tray is replenished daily.
It also comes with an in-room Keurig coffee maker instead of the lousy Cuisinart pour-over pods that are in the regular rooms. I'm a HUGE coffee drinker, so I'm considering doing it on my upcoming September stay.
The cash upgrade price is listed as $17/night on that reservation. That seems expensive, but the store in the lobby sells candy bars for $2-3 each. So, you could theoretically end up close to even.