Create obstacles

One chasing the other by running in place. The person in the front mimes various terrain changes like jumping over things, going under things, ducking, climbing ladders, falling into holes, etc. The second player copies what they are doing as if they are running over the same terrain. 

Build Obstacle version
Player 1 builds obstacles in front of the chaser and the chasing person needs to figure out how to get through them. For example, Player 1 may pantomime nailing up a bunch of boards in front of Player 2. Then Player 2 gets a sledgehammer and busts through them. Player 1 advances and pours buckets of sticky substance on the floor, player 2 sprays oil on their feet and walks over the ground w/out sticking, etc. 

Variation
Do in slow motion (this is somewhat necessary in build obstacle version). 

Application (non-improv) 
Have the player assign meaning to the obstacles and/or to the quality that they are going to access in how the move past the obstacles. For example, the obstacles could represent real concerns, inner thoughts, fears, challenges, etc. For moving through obstacles they could play with a particular quality (being creative, being playful, being powerful, being bold, etc.)