Where's the best spot for seeing the Aspen turning colors in or near Boulder?

Gold Hill! Just about 35 minutes up the canyons from Boulder and you start hitting the high foothills where the aspen right now are about half way "done" already

Rocky Mountain National Park (RMNP) is a great day-trip from Boulder to see the leaves changing in the Fall and at the same time, you can see the elk bugling. Herds of elk come down into Estes Park which is the gateway to the RMNP. It costs $20 per car to enter the park or you can buy an annual pass if you plan to go several times a year. The scenery is absolutely spectacular and there are loads of great hikes to do in the park. Of course Aspen itself is not to be missed, but it is not a day trip as the drive there is 4 hours from Boulder.