Go for a Weighted Walk
How do you make the "dreadmill" less dreadful? One simple way is to grab a weight, crank up the incline, and do some farmer's-style walking. You can use a dumbbell, a weight plate, or a kettlebell. (Weight vests are cool too, but this method is more challenging and has more benefits.)
Hold the weight any way you can: up on a shoulder, against the front of your abs, in one hand down by your side, and shift positions whenever you get tired. You won't be using huge weights on this, you're on a moving treadmill after all, but this is a simple way of getting loaded carries into your workout.
Go Steep with 25-Pounds
Use a steep incline, but keep your speed moderate. Walking fast with a 25-pound plate in your hands is one thing; running with it is quite another. Two-minute intervals should be about right on this one. If that's easy, go heavier and steeper before you go faster.