I got fed up with the same thing a while back myself. I now use light squats, usually 25% of my 1 RM. I do sets of 25. They usually get my heart rate up. I do about 4 sets of these. Other days I use overhead pressing, with the same type weights. Bench the same way. One of the additional benefits, is a great pump.
Try kettlebells, lifting for time. For example, one-arm snatch, in sets of 5 to 10 reps, switching arms, and continuing, for 10 to 20 minutes, trying to increase the amount of reps you perform each cardio workout.
I also like to just shoulder and drop a sand bag non-stop for ten minutes.
I also created a circuit for the park. Run 100 yards with sand bag. Sprint back without them. Lift Kettlebells for set number of reps. Sprint back to get sandbag. Carry back to Kettlebells. Etc. Its a gaser.
One AWEFULL but effetcive favorite of mine is to do weights in an HIIT fashion. I will pick say three moverments I can easily do with a light weight. or body weight. Usually one push one pul one legs.
For an example let say I choose pushups, BB Bent over rows and DL's. No I will simply jog or jump rope etc in place for a warm up. after five minutes I will begin to cycle these movements On andd off. with the moderate paced activity during recovery. So I would be jogging in place jumping wghat not. then Grab the bar and Knock out DL's for 30 seceonds, as fast as possible giving it your all afetr 30 seconds back to the jogging for 90 seconds, the Hit the pushups for 30 seconds, then back to the jogging for 90. then hit the rows for 90 seconds, rinse and repeat that for three sets of each and do a cool down of five minutes or so and you have had one hell of an interesting and effective cardio session.
One thig to remember is to keep the weight LOW on the movments.
Try sandbag lifts. I do it once or twice a week, just get a heavy sandbag and lift it up to your shoulder. Switch sides every other lift to keep things even. I do them by either going for a total number with as few sets as possible (high reps) or for a total number with smaller sets but very brief rest periods. Either way just try to get more lifts in less time each time. A half hour or hour of these really gets the heart going, and it is just plain good old-fashioned GPP.
Anything similar, from chopping wood to Tabata squats, can be good cardiovascular exercise as long as you keep the intensity high and the rest brief. When you feel the heart pounding, that means it's working.
The actual study by Dr. Tabata showed that it improved aerobic capacity by 14% over low/moderate cardio and increased anaerobic capacity by an unprecedented 29%. It also burned 9x more subcutaneous fat. It's been a while since I read the study, but I think those numbers are accurate.
I do squat/curls, where I squat with two dumbells and on the way up, do a curl. There are endless variations, like the ones that Dan Johns mentioned. My heart rate gets up to 180+ when I am done. I have recently upped the sets to 10, instead of 8. I found that when you can't go beyond 80-85% of your average, it's time to stop.
Tabata anything and that will be your workout for the day. I like to do "coreblasters", something like kettlebell swings. I've made a couple of them, where you make a t-handle out of pipe and put 25# plates on it. Dan has pictures somewhere at his website. I do 90#, about 1/2 my bodyweight. I forget exactly how many reps I do, but it always kicks my butt.
Why doesn't anyone mention sport? If your board with a treeadmill find some game or sport you can do- its the fall so the opportunities are there. I know that not everyone is athletically inclined, but is an option that brings you into the outdoors and often interacting with other people.
Plus you get the benefit of seeing the effects of your efforts- speed, power, or the sheer inability to be moved through any amount of force.