The best replacement for treadmill is elliptical.

The best alternatives to treadmill are rated as following:
★★★★★  elliptical
★★★  bike
★★  running
★★  stationary bike
★★  track
★★  outside
★★  outdoor running
★★  exercise bike
★  walking
★  road
★  road running
★  elliptical machine
★  rowing machine
★  elliptical trainer
★  stairmaster
★  treadclimber
★  cross trainer
★  outside running
  running outside
  elliptical weight loss
  bicycle
  elliptical calories
  incline trainer
  elliptical workout
  gym membership
  street running
  indoor track
  ski machine
  elliptical for toning
  run outside
  jogging
  outdoors
  stair climber
  real running
  chemical stress test
  stepper
  elliptical better
  treadmill
  spinning
  cycle
  running outside calories

Your alternative for treadmill is not listed? Suggest!
Speaking about treadmill, one probably shouldn't forget about incline.

Other search terms related to treadmill and their ratings are:
★★★★★ incline
★★★★ ifit
★★★ weights
★★ desk
★ thallium
running
You have in mind a companion for treadmill and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of treadmill in all years. Click on graph for more.
treadmill
elliptical
bike