This is an expanded version of my comment on the original post, so if some of it sounds like you’ve read it before, you probably have. I've explained every one of these points to at least one person in real life, so even though I'm sure some of them seem like common knowledge, not everyone knows.
Tickets: always check StubHub. There have been some terrific bargains there (not as many since they raised their fees substantially but still some), and the more flexible you are, the more likely it is you’ll find one. Many sellers have set it up so the ticket prices go down as the date and time of the game gets closer, so it becomes a game of chicken, but if you’re up for a last minute trip to the ballpark, you can do well.
Join the Rockies’ email list and/or check their home page. They have frequent Blake Street Steals or other specials (like $2.50 tickets in the upper deck to celebrate Todd Helton’s 2500th hit) – the deals often sell out , so check early.
Seats: there are no bad seats in Coors Field, there are only different game experiences. Where you want to sit depends on what kind of experience you’re looking for.
My favorite price/performance seats are in the lower upper deck, call it roughly sections L318 – L340. There's an excellent view of the field and the sunset.
If you do get seats on the ground(boxes), you can make your way through the seats to stand along first base line to wait and see if any players will be out signing. Bring something to sign and a pen if you want a special pen used – the security guys will hand the players a sharpie when they see a player start signing.
Even if you don't get an autograph (and chances are you won't), the other folks waiting with you are often have great stories. And while some of them are pros, I've found most of the pros are amazingly willing to help amateurs like me.
If you are going to a day game, there is a reason that the seats in higher numbered rows along the third base line are priced higher on the resale market than the seats right up front along first base line. The former seats will be in the shade, and the latter will be baked by the sun. Bring sunscreen (lotion, no aerosols, aerosol sunscreen is not permitted in the stadium and will be held at the gate for you to pick up after the game when it's too late) and be aware you're going to roast. If you don't mind that, you can get some really good deals.
Food and Drink During The Game: The Sandlot brewery brews beer right there at Coors Field. I'm not sure what all they're pouring this year, but in the past they've had a very nice red ale.
The food at The Sandlot is also pretty good – they do a loaded nachos with pulled pork and many other good toppings other than the usual canned cheez sauce and pickled jalapeños. The onion rings at the Helton Burger Shack are good, but I find the burgers lacking. On the upper deck is Biker Jim's Gourmet Dogs – if I recall correctly they have only a subset of their regular menu, but they do have some of the interesting sausages and the cream cheese and onion topping. (I've since been informed they now also have a cart on the main concourse.)
However, since Coors Field allows you to bring in outside food and sealed, non-alcoholic drinks (subject to restrictions), we usually pick up a couple of sandwiches from the Jersey Mike's on the way to Light Rail, and buy a frozen bottle of water from one of the many vendors on the walk from Light Rail to the stadium, and then just buy a beer once we get there. It's a lot less expensive so we can go to more games :). And we often get longing, hungry looks from the folks inspecting our bags :D.
Also recommended: the chocolate berries and brownie bites on a stick (sold at carts on the concourse and sometimes via vendors in the stands) and the cinnamon roasted nuts (on the concourse, third base side).
If you're on the upper level, the better beer choices are at The Clocktower (behind section 330).
Before the game: Get there early and watch batting practice. Spend some time wandering around the concourse – there are TV screens and promotional giveaways and lots of stores selling all kinds of Rockies branded stuff you probably don't need but will want anyway and game used items. Coors Field was designed with a traditional, "old-timey" feel, and it's fun just to soak that up for awhile.
Go to the upper level and enjoy the spectacular view of Denver from the upper level concourse. There are also picnic tables up there if you don't want to eat at your seat, with the same great views. Sunsets are often jaw-droppingly beautiful, and the higher up you are in the stadium, the better the view.
After the game: we tend to go home :). If we do go out for a bite to eat later, we usually go to places somewhat further away from the stadium. I like Osteria Marco or The Market in Larimer Square, Wazee Supper Club, or the places in Union Station, like The Kitchen Next Door. You're more or less in downtown Denver, so there are lots of options.
A lot of the restaurants do close at 9:00 pm or 10:00 pm duuring the week, so check to see if they're still open before you go, especially if the game runs late. Sadly, Denver isn't really a late night fine dining kind of city.
Assorted other thoughts: On the Rockies' website is the Coors Field A-Z guide. It's actually quite useful, and a good place to check first if you have any questions.
When my sister and her husband from Kansas came to visit when the Rockies were playing the Royals, she asked me if it would be ok if they wore their Royals t-shirts and hats. I just laughed and said it's fine – they would not be the only ones so attired, and no, they wouldn't be bothered. They weren't, not even when the Royals won. For the most part, Rockies fans tend to be a pretty mellow group.
My brother was out here on business in April a couple years back, so we all decided to meet up at a Sunday afternoon game. He asked, "what's the weather going to be like?" I answered, "It could be 80F; it could be snowing.". The Friday before, it was 80F; the game we were supposed to go to was snowed out. So we all ended up at Wynkoop drinking beer and eating burgers.
The moral: This is Colorado: expect weather. It can be sunny and 72F when the game starts and 35F when it ends. Or it could rain. Or snow. Or hail. Or none of the above. It never hurts to pack a hoodie and a folding umbrella if there's any question at all.
I'm not sure if I need this, but just in case: Disclaimer: all the businesses mentioned here are places we've patronized and enjoyed. I have no relationship with any of them other than that of a satisfied customer.