If you're looking for a meeting place or a good stop around ½ or halfway between Denver and Boulder, you can try Broomfield, CO, which is about 18 minutes north of Denver, or about 17 minutes southeast of Boulder.
About ¼ or a fourth of the way, you can stop at Westminster, CO. This is about 10 minutes north of Denver.
About ¾ or three fourths of the way, you can stop at Louisville, CO. This is about half an hour northwest of Denver, or about 10 minutes east of Boulder.
If you drive straight through without stopping at all, the total distance is 29 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 35 minutes.