I like using the bike for tempo work and we have an Schwinn Airdyne at our facility and allows the incorporation of arm movement into the bike workout. I have also personally found that moderate load single arm KB snatch has been a nice alternative especially if my goal of the session is to work the aerobic system. As an example
the other day I did 30 seconds run on treadmill, 30 seconds rest to move to next station, 30 sec KB snatch, 30 sec rest, 30 sec airdyne, 30 sec rest, 30 sec KB snatch, 30 sec rest and repeat x 10