I'm from CO, born and raised in the High Country (8k ft and higher).
Two suggestions for you:
If you're looking for a quick, but beautiful ride through, you're going to want to drop down a little south and hit I-70 through Grand Junction. Stay on that highway all the way to Denver. Word of caution: the highway after Eisenhower Tunnel shortly after you pass Silverthorne can be a bit intimidating especially when you have stupid drivers in the area. Use caution b/c that's a fairly busy road and stupids from Denver tend to clog it some. While you're in the area, if you want to make sure your bike is doing good, stop by in Silverthorne. When coming from Westbound traffic, you'll have a turnoff on the right hand side that gives you the option to turn right or left. Go left. You'll pass (all on the left) a 7-11, a Target, and shortly after that (before the final gas station) is Silverthorne Power Sports. Tell them that Adam (Ray's son) sent you there. If you can, talk to Mike, a really good guy. This is also where I bought my FZ6, and they were REALLY awesome.
Once you hit Denver, you're outa the woods...so to speak.
Option #2: For a relaxing, not so quick, absolutely gorgeous ride, hop on hwy 131/40 going through Steamboat Springs (Use caution at Rabbit Ears Pass). I hate Steamboat, but you're not going there for the city are ya?

Stay on 40 and you'll go through a small town with one stop light and a helicopter in front of the Highschool - Welcome to my old home: Kremmling - btw, that road you could take to get to Silverthorne Power Sports is hwy 9, and leads right into Kremmling too, in case you still wanted to do a quick check up on the bike. Stay on 40, keep going until you get shortly past Hot Sulpher Springs. There's a turn off on the left at the same spot a gas station is. GAS UP HERE!!! It's cheaper than what you'll see coming up. Take that left I mentioned, this is Hwy 34, and it takes you through Grand Lake (gas is EXPENSIVE!!), and then through the Rocky Mountain National Park. Careful at the peak. I kid you not, you'll want to dress warm for this part, and use every cautionary trick you have. This is litterally a very thin road with a rock wall on one side of you, and a sheer drop hundreds of feet down immediately on the other side of you. BUT - this is by far one of the best routes to take through CO. And please, respect the altitude by not getting off your bike and then walking out into the hills for a little bit. It takes so long, and it is so difficult for things to grow there that when you take a step, whatever was under your foot is now dead. Kick back, enjoy the beauty of it, and take pictures!! You'll see some amazing wildlife if you're lucky. BTW, Moose tend to hang out around Grand Lake, and for sure you'll see several head of elk through the park. Stay on the main road, gas up in Estes Park - major tourist area, and the golf course tends to get littered with elk too! Stay on that road, and you'll enter into a canyon that leads into Loveland - one of my old stomping grounds and 20min south of Ft. Collins (you said you wanted to go there). You can gas up in Estes, but would be better if you did it in Loveland at a Diamond Shamrock that will be on your right as you enter an older part of Loveland. Staying on that road will lead you through town, past Lake Loveland, over a bridge, through newer parts of Loveland and all the way to I-25 where you can head south towards Denver. If you're hungry, shortly after that Diamond Shamrock is a small place called Justine's Pizza, owned by a family that are good friends of mine, and probably the best place to get pizza there. If you don't want pizza, then stay on the road until you hit the outskirts (shortly past SAM'S club) and you'll see McGraffe's on your right. Tim and Mike (different one) are the owners, and serve up some dang good chow. There's a sports bar in the place too with 2 or 3 pool tables.
To avoid Denver traffic, I suggest taking either I-270 or I-225 (depending on where you come from).
As far as weather in late may: should NOT be an issue except on that pass going through Rocky Mountain National Park. It'll be cold up there, but hey, you're up in altitude and this is CO - bring layers

Now, we HAVE had a FEW winters where snow was still pretty heavy up there. Before you hit that part of the country, send me an e-mail at
[email protected] and I'll contact mom and dad to see what the weather is like up in the high country for you so you can get some decent information from some locals.
If you need info about Ft. Collins, let me know. =)