From a legal point of view, a few tidbits: marriage licenses are $30, and are good for 30 days. You do not have to be a CO resident. You and the Fiance have to show up with ID, and swear you’re not related to each other (I believe CO does allow 1st cousins to marry.) Also, you do NOT need an officiant—CO allows the bride and groom to self-sanctify, which means you and the Fiance sign the license, and that’s it. You’re married. (If you do use an officiant, it’s the usual guidelines of who can officate…which doesn’t really matter, since we had a friend ‘marry’ us, then the two of us just signed the license.) Awesome. Something to think about if you’d like to bring along a friend to perform, instead of a stranger.
All of RMNP is beautiful, but my favorite part to hike on the east side is Wild Basin. I particularly love the Finch Lake Trail—it will take you through an almost 30-year old burn (burns recover VERY slowly in CO) which gives you some astounding views, all around, but also of Long’s Peak (14’er). If you take the Peak-to-Peak highway from Estes to Denver, you might be intersted in stopping at Brainerd Lake Recreation Area, Nederland (Hessie TH, Fourth of July TH) or Rollinsville (East Portal TH) to do some day hiking.
September is lovely–second or third week is usually the aspen peak. Varies a lot throughout the state, though, depending on altitude and aspect (direction the hillside is facing). RMNP does not have a lot of aspend stands, so if you are interested in the fall colors, you might want to consider heading out I-70 towards one of the ski resorts for a few nights. The world’s largest hot springs is in Glenwood, CO, about 3.5 hours from Denver. It’s nice, but not rustic—-two giant olympic sized pools with hot mineral water. It is right on the way to Aspen, though. For a more rustic hot springs, try Hot Sulphur Springs, near Winter Park (another ski resort)–they have 24 or so small tubs outside ranged up and down the hill. My favorite is Strawberry Hot Springs in Steamboat (yet another ski resort, but not out I-70)–very rustic, in that there are 3 different temperature pools built into the rock, and how hot they are is deteremined by how much water from the adjacent river goes into each. They also have VE”RY rustic camping, if you’re interested. That’s about a 3-4 hour drive from denver.
You mentioned kayaking–not a lot of whitewater here by september. All our whitewater is snowmelt, so May and June are the months it’s really going gangbusters. The dam-controlled rivers are usually shut down by that point, too. I believe August is when they close the dam that feeds the Arkansas, down south. Still water, just depends on your level of kayaking skill and what you’re looking for.
What to see in Denver? There’s a zoo (nothing super-special), Museum of Nature and Science (museum seems a bit dusty, but has an IMAX), Art Museum, History Museum, Museum of Modern Art. Denver Center for Performing Arts (DCPA) is quite large and has 7 theaters, hosts national touring shows. Ummmm….one of the state capital steps has a medalion for one mile above sea level. The row of purple seats at Coors Field also mark the mile-high elevation (home plate is actually below street level). There is a B-cycle (rental bike) hub at the City building (WEbb building) if you want to tool around town that way, with more bike stations planned for next year, I believe. Santa Fe (street) Arts District hosts first fridays art walk every month—all the galleries have open houses. Tattered Cover bookstore downtown in LoDo (lower downtown) is a booklover/browsers delight–three floors of nooks and crannies.
September is a good time for mountain biking. You can try the front range Jefferson County (JeffCo) parks, or head out to Buffalo Creek–that’s a good area with some beginner friendly bike trails. Golden is on your way, and has four bike shops—you could rent a bike there.