Places to formally camp alomg the metro trail system include Chatfield State Park, Bear Creek Lake Park, Cherry Creek State Park and ???
You can certainly plot a series of overnight camping stops along the metro trail system, which is 100's of miles long.
Here is a map of the major metro area trails. BTW, you can ride to the west side of Castlewood Canyon state park entirely by the Cherry Creek Trail and about 3 miles of Castlewood Canyon Road - little traveled - there is one mile of dirt - but easiy travelable - trail south of Parker.
For example, you could start from Chatfield, take the ighline Canal Trail to the Cherry Creek Trail, catch the trail to Cherry Creek State Park, spend the night (get reservations) then head to Confluence Park, then south on the Platte River Greenway to the Bear Creek trail and to Bear Creek Lake Park, spend the night, then back to Chatfield. There is just a bit of climbing getting into Cherry Creek State Park and into Bear Creek Lake Park over Mt Carbon, but nothing bad. Lots of other potential routes of 50 iles or so.