If you are looking for something fun that is outdoors, we would suggest planning a trip to Barr Lake! This is a great place to walk around and enjoy the views of the lake, plains and mountains. They are well known for their Bald Eagle nesting area, a rare sight to see. I believe it is a $10 entry fee but if you have a state park pass, its free.
The Arsenal was previously a chemical weapons manufacturing center but has been converted into a wildlife refuge. It is known for its population of buffalo and the scenic 11-mile wildlife drive. If you feel ambitious it has a 21-mile trail around the premises that you can walk, run or bike around. It is free as well!
If you are looking for something close to the mountains but still something not too far away, we would suggest making a trip to Golden! The town is super fun to walk around, and Clear Creek is beautiful to walk next to. The School of Mines is located there as well, and it is nice to walk around campus.
The oasis is a fun family water park in Brighton. It is nice to go on a hot day to sit by the pool. There is a lazy river, slides, a playground and a rock wall.
Frisco is one of our favorite places to go in the mountains! The small town is fun to walk around, and Lake Dillon is great for fishing. Frisco is close to the town of Dillon, Keystone ski resort and Breckenridge. They have a lot of fun things to do in the summer.
RMNP is known for its beautiful hikes and easy accessibility to the public. It is fun to go and drive around and get a real feel for the mountains. There is an entry fee but if you have a national parks pass it is free.