I'm going hiking in Colorado in July and will need to rent a car. Looking into it today I saw that rentals at DEN have really shot up and cost twice as much as car rentals downtown. Around $100 per day at the airport for a compact. I know there's a shuttle train from the airport but I don't know where it stops and if it even goes downtown. Can anybody tell me about a shuttle bus or give me another idea about how to get from the airport to downtown?

If you have a particular car rental office in mind, you can use the RTD's trip planner. It might be a bus/train combo.

https://rtd-denver.com/app/plan

If you're a Costco member, you might want to check their rates.

And it has been reported here that even if you rent off site you can return to DIA. I've never done it but you might check.

The A line train goes from the airport to Union Station in downtown. It's about 4-5 blocks from there to Budget. I believe there are also Enterprise and Hertz locations downtown but I'm a Budget member so I don't know the exact locations of other rental companies. I've rented from downtown and returned the airport probably a half dozen times. It's usually 40% cheaper with no drop off fees. It is probably about a 10 minute walk from Union Station to the downtown Budget location and it kind of sucks with a large suitcase, but it's worth it for the money you save.

My next trip, I'm renting from a different off airport location and taking an Uber from the airport to there - it's about 3 blocks off I-70 so it will be very easy to get to. It's $230 for a week at that location vs. $400 a week from the airport and I can drop off at the airport for no additional charge.

Take the A line from the airport to Union Station (end of line). $9.50 and about every 15 mins. Union Station is somewhat on the north end of the downtown area. Depending on where your rental office is (check a map) you can either take the free Mall bus on 16th street on the west side of the station or the free Green line running down 19th. You catch that on the lower level of the Union Station. Those buses run on short intervals - in the range of 5 to 10 minutes depending on time of day.

Thanks to all of you for your help. The A Line advice was perfect. I googled the situation and found everything on my Denver map. This is going to save me some real money.