I am heading to Denver in late October - staying at the Hotel Magnolia - any opinions on this hotel? Also, thinking of adding a day or weekend at the end to go to Boulder and RMNP - would this be a good time to see RMNP?

Good hotel choice - you won't be disappointed.

Boulder should be beautiful that time of year. RMNP will not be busy... but Trail Ridge might be closed when you go. Check with the park to be sure.

I should add... Boulder is always beautiful in my opinion. Weather can get chilly in October though... and it could be wet. Be sure to bring layers.

Most certainly RMNP will be gorgeous and empty of people that time of year; you'll see a lot of elk, whether or not Trail Ridge is open. You could be in the 70s, or it could snow--you never know! Yes, bring some light layers (and a hat and gloves) just in case.
Heard nothing but good about Magnolia.

Late October isn't the ideal time for RMNP, but if that's all you have, then you might as well go for it. I would expect a fair number of the aspens will have lost their leaves by then, so there probably won't be much in the way of fall color. However, if there is some snow, you'd get to see a side of the park that most people don't. TRR may be closed, but the lower roads are open all year (with occasional closures until they can get them plowed).