Disclaimer: While I am a certified personal trainer, this advice is generalized and should be discussed with a doctor or in-person trainer who can assess your fitness status if there is any doubt about your condition.
(1) Mess with the incline
Is your 30 minute jog on the treadmill getting a bit dull? I'm pretty sure that doesn't surprise anyone. To change it up (and to make you more fit, simulating outside running) continually mess with the incline on the treadmill during your run. Keeping the incline at 1.5-2.0 instead of 0.0 can be very effective. Then throughout your run, just move it up to 5.0, then down to 3.0, then ramp it up. Don't put your phone in front of you with some Pinterest "HIIT" treadmill workout where you go cross-eyed trying to read it. Just be random about it, as if you were out on the street running.
(2) Combine your weight movements
Do you do arms one day and legs the next? Change it up. Do squat press, where you hold 2 dumbbells or a barbell on your shoulders and squat down. As you return to standing, raise the weights for a shoulder press. Combining the two exercises will more effectively work your core and back muscles.
Other examples? pushup-row, where you do a pushup (on your knees is fine!) on 2 dumbbells. As you return to straight arms, shift your weight and row that dumbbell up. Want more of a challenge? Lift it all the way up, like in a yoga sequence, twisting your body towards it. I challenge you to come up with other combined exercises!
(3) Swap for speed
Remember the 30 minute jog from number (1)? Switch it up. Two or three days a week (if you do 5 day cardio) - or 1-2 days if you do 3-day cardio - go for speed. Instead of a 30 minute jog, do a 10 minute "sprint" at 3.0 mph faster than usual.
(4) Switch to body weight
Add in some cardio by doing body weight movements. "Wait, what?!"
That's right! Switch to bodyweight squats and not only will you improve your form and feel your abs screaming, but you can do them faster as your form improves, giving you a dose of cardio.
Do you do lunges? Try it with your arms overhead. Doing any exercise with your arms overhead makes your heart have to pump more blood up there, increasing your cardio burn. Want to improve your balance? Don't let that trailing leg touch the ground after your lunge, pull that knee up to your chest and hold! Then repeat.
(5) Try plyometrics
Plyometrics are explosive body movements that give your muscles a whole new workout because of the anaerobic capacity engaged. Some examples?
Jump squats. Squat down (no weight) and instead of coming back to standing, jump up. You'll look like a star!
Lunge squats. Lunge back, then tuck your back leg up to your chest and jump on one leg, arms overhead.
Lunge switch. Lunge forward, and then jump, switching your legs in the air, landing with the other foot forward.
Pushup with clap. On your knees or toes, do a pushup, but as you actually "push up", give an explosive push so you can get some air and clap your hands.
I know I haven't included any photos here, so if you have questions just let me know! Have fun!