Depending on your route and time of day--Bonfire burritos in Golden has the best breakfast burrito around (in my opinion). They have become real popular in the last couple of years so their lines can be pretty long. I find it best to go soon as they open which is 7am. Golden is about 30 min west of downtown Denver.
Voodoo doughnuts is worth checking out if you have a sweet tooth. They have a wild selection of donuts. Wild. You just have to go and experience it. They even have vegan and glutton free options if that is your diet.
Another pretty good place is the Cherry Cricket in downtown Denver(they have two locations). It is a burger joint with craft beer. They have a pretty good menu variation if your wanting something other than a burger.
If you are more into "fine dining" then Fruition Restaurant off of 6th Avenue in Denver is incredible. All of their menu items are fresh from local markets and local farms. To warn you--the menu is pricey (at least to me). My wife and I have gone there a few times and our bill is usually in the $300-400 range (special occasions only for us).
I haven't been downtown Denver in a couple of months, so I cannot comment on what the "mask" situation is like. I know in my county, they are STRONGLY encouraged. The county in which I work masks are REQUIRED. You might as well wear it, no sense in risking your health (lets keep politics and freedom out of this discussion). There is a rise in cases locally, just FYI.
If you are wanting to swing into a marijuana store, bring your ID. They wont sell or let you in without one. Typically you walk into a room where they scan your ID, and then you sit in a waiting room (just a few minutes). Once they are ready, they'll inform you and you walk into a separate room to browse and purchase. You'll deal with one individual and your options are within their section of the counter/store. Feel free to ask questions, they're helpful and deal with out-of-staters regularly.
As far as places to avoid--hard to say during these crazy times. Again--keeping politics/race/religion/any other touchy subjects out of it--Denver had a young black man die at the hands of police last year and lately there have been a lot of protests. They are peaceful, but when the sun goes down there is usually some uneasiness and may be rioting. We were at a vigil a couple weekends ago and it was very peaceful with violinists playing to remember the life that was lost. Around the time the sun went down, police came in riot gear and teargas was unleashed. Just an FYI. Again, lets keep the politics/religion/race out of this discussion. I just want you to be aware of what may (or may not) happen.
Colfax Avenue can be interesting. I have never been harassed or anything like that, but there are a number of homeless camps around and people of all diversities and state-of-minds. Don't be surprised if you have someone asking for change, come across someone sleeping on the sidewalk, or a rambling incoherent individual, or just plain old vomit on the sidewalk. People will for the most part, stick to themselves.