Running sprints is by far the king of outdoor conditioning.  But when the weather takes that option away, there are still plenty of alternatives.  Jumping rope at an intense pace is an excellent supplement.  You can get a quality rope for around $6 dollars.  Far exceeding anything a treadmill can offer you.  Mix in bodyweight exercises like burpee's and mountain climbers and you have a quality workout.  Further this by adding in an interval time, say 30 seconds each exercise with a minutes rest. Or the ever growing (and for good reason) tabata protocl. (20s work 10s rest x8)  Stack blocks of 4 minute tabats ontop of eachother to truly push yourself.