Welcome to our dapper world! You're going to have so much fun! You are so right to put practicality and comfort at the top of the list. Weather dictates both priorities, so keep watching the forecast for the days right up to your trip dates. We've done Dapper Days in triple digit heat and in cooler/damp weather, too. Very different challenges for both. No matter the weather, wear comfortable shoes! Some people keep heels in their tote bags for photos and slip back into their flats afterward. Ballet flats, Croc flats (ballet style), mary janes, low heels, saddle shoes, etc.
You can dress up super fancy or more low key -- just make sure that you are not in pain and able to move freely and safely throughout the day. Remember that "dapper" is a particular look/style, not linked to one particular era. There are many different ways to dress dapper (both modern and vintage)! If you want to attend the expo in the DLH and hit the dance floor, make sure that your clothes move freely and that there won't be any wardrobe malfunctions. For rides while wearing a dress, if your dress is shorter, wearing cheer shorts or bike shorts underneath can bring much peace of mind and allow more freedom of movement, especially while getting into and out of low ride vehicles. For the carousel, CMs will ask that you face inward (toward the center) if you choose to ride sidesaddle while wearing a dress or skirt. (Modesty reigns in King Arthur's court!) For longer dresses and skirts, just make sure that you can get in and out of ride vehicles without getting your skirt caught in the ride door or in your heels. If your outfit is very full or complicated (i.e. lots of buttons and hooks and layers), using the single room restrooms can be much easier than trying to squeeze into a regular stall. I usually bring a cardigan for daytime and put a jacket or coat in a locker for later, if needed. Layers are your friend at DLR most days, but especially on Dapper Day. And look for fabrics that breathe! Vintage outfits are fun, but vintage fabrics don't always cooperate at DLR, especially if you'll be in the parks all day. I like to keep makeup more on the simple side. The complicated looks can look great in the morning, but look bedraggled by the end of the day. I'd rather keep the touchups easy and not have to be looking in a mirror or running for a restroom all day long. There are many much more fun things to do on Dapper Day!
And if you have room in your bag, keep a small emergency bag of safety pins (various sizes, especially small ones), band aids, body glide, hair pins or clips, tissues, etc.) -- whatever you think you or your outfit might need in a pinch. I haven't had to use mine very often, but I have come to the rescue of a few other people!
