Denver Dining Casual & Unique?
Depends on what you're looking for. Panzano is low key with fabulous food. There are several brewpubs with decent grub and great beer - Wynkoop Brewing, Breckenridge Brewing and Flying Dog. If you go to Flying Dog call ahead and see if you can tour Stranahan's Distillery next door and sample their Colorado Whiskey. Surprisingly good.
I'd go to Prima in the Hotel Teatro and hope for good service. Food and atmosphere are terrific. Panzano IN the Hotel Monaco is, IMO, a whole lot better than The Broker. The Broker's gimmick is their shrimp bowl, which has become increasingly mediocre. Rioja, Bistro Vendome and the Capital Grille (all Larimer Square) are good. They serve, respectively, Italian, Mediterranean, French and steakhouse fare. If "downtown" includes LoDo, and if "a very lovely friend" likes Italian food, Venice and Via (both on Wynkoop) are excellent.
If you are looking for someplace really interesting with tons of history and a great ginourmous signable guest book... The Broker is your place on 17th near the Hotel Monaco.
In the ballpark area (22nd and Larimer) Snooze does a great lunch. Hipster central, but the food is great.
Maddies House of Mirrors (a former brothel) on Market street might still be open for lunch, and I have heard but can not confirm their lunches are good.
Il Fornaio is great on Wazee and for a nice change of pace you could always try Gumbos on 16th Street ( I have never had a bad meal or bad experience there, but make sure to put in a reservation...lunch fills up quickly).
A little further in uptown you could always go to Stuebens which is delicious and again, great atmosphere.