First, you don't need to take E470 to get the coupon - just print it out (or have it on your smart phone) and you get the credit. And, yes, if you only go to DIA on weekends E470 isn't a better route.
The biggest hassle with Canopy is that you have to take a shuttle to/from DIA - and this is a factor (add 20 minutes each way on average) so all things being equal I prefer the DIA garage. But things aren't always equal, both in terms of cost and in terms of getting a full EV charge. The valet actually minimizes the hassle otherwise - you drive up, they take the car, and you get the priority pickup for the shuttle meaning you are the last stop when taking people to DIA and first stop when arriving back at parking. When you get back the shuttle driver radios ahead so your car is ready to go when you arrive. If you use the valet they'll charge the car when needed then park it elsewhere so for 3 weeks is fine - they'll even do an 80% charge to start then if you call to remind them they'll do the top-up a few hours before you arrive.
AFAIK there are electrical outlets somewhere in the DIA garage because the article which announced the L1 spots specifically mentioned that people were charging using the various outlets there. However, I've never seen one although I've looked a few times.