Put together a variety of outdoor pioneer activities such as a log sawing, stick pulling, and leg wrestling contests.  Have someone teach the group how to some old-fashioned dancing and invite everyone to show up in pioneer garb (bonnets, suspenders, etc.)  And an event such as this is the perfect opportunity to cook some Dutch-oven favorites like cheesy potatoes and cobbler.

Other pioneer activity ideas that could be used to help plan your activity:

A Journey to the Valley
A Pioneer Trek
Making Butter
Picnic in Nauvoo
Pioneer Treasure Hunt