Using the out-of-doors as an extension of your classroom is a good way to get your students up and moving, yet it is even more critical to give them experiences that will help them understand, appreciate and care for nature. Learning STEM skills and content in the field, in their world, is good practice. There are a multitude of Citizen Science activities that would be appropriate.
Squirrel Mapper has genetics and evolution connections, alternatively you could direct your students to observe squirrels in your school yard, or a nearby park, and then have the students come up with their own questions that the class could develop into driving questions with your guidance As a bonus, here is a funny and short squirrel video that you can use to spark some questions about squirrels even before going outside.
If birds are of interest to your students—or if you would like to encourage that interest—check out The Cornell Lab of Ornithology (at Cornell University) for their ideas.
Connecting to the 3 Dimensions of the Next Generation Science Standards
The NGSS Practices