Step Ups. They're quick, not easy and work the core and cardio. Start out without weight with the basic exercise. 10 Reps per leg. Then when your comfortable with that it's time to move up. After stepping your leading foot up, instead of bringing your trailing leg up to the surface with the leading leg, bring it from the floor while stepping up and bring the knee of your trailing leg up into your chest and then step down. 10 reps per leg again. After a few days when that's not kicking you in the butt, grab a pair of dumbbells' to add weight. Increase the weight as the days go by when your feeling comfortable. I absolutely hated this exercise starting out as a power lifter, but I quickly found my dexterity and endurance improve as well as my energy level to make it through the rest of the workout. I don't even think I could make it through all the reps to start with the first few days and I would go off to the side and keep my hands reached toward the sky and jump as high as I could. They were one of my essential exercises no matter how goofy I looked doing them. I have this for my step ups to adjust the height with additional risers since I'm 6'7 and my wife is 5' nothing. It supports a lot of weight, great traction and I have yet to break it in 7 years.
The Step
Video of basic motion
Another is Battle Rope. This also helped my endurance/cardio work. You would be surprised if you stood straight up, bend your knees so your shoulder is over your toes and then just pump your arms to move it like your jumping rope or beating a drum as fast as you can for 30 second intervals over 2 minutes will improve cardio. I've got these which are super nice but you don't need to spend that much. You can find a used battle rope at a used sports equipment seller for $20. Just make sure you anchor them down well. I tie mine to a bottom support of my work bench.
Gronk Fitness battle rope
Same with like a wind bike, 2 minutes, 30 second intervals of pumping fast as you can and 30 seconds of recover but keep the limbs moving until the next 30 second drive. I don't have one but I wish I had a rogue fitness fan bike.
Aside from a few resistance bands I picked up cheap (
(Black Mountain Bands) the only other thing I have is a PowerBlock Bench, Urethane Dumbells with stage 1, 2 & 3 kits, Kettlebell conversion handles for the power block and curl bar for them and I'm pretty set if I want a full workout or just do whatever suits you. I used this in my days when powerlifting competitively and needed a second workout or just could not make it to the gym for the day.
Truth is you really do not need much for equipment if any for decent fitness. If you have a basic understanding of the movement you want to mimic, you can always add resistance or find ways to make it more difficult. Power lifting taught me that in strongman all you need are big heavy rocks or a hardware store. About the only thing I went to the gym for was use the Bench Press, squat rack, pool and sauna. That reminds me, if you have a local YMCA the pool is a great place to improve endurance and cardio work. Simply tread water for a specified amount of time, go in water about rib deep, drop under water to a fetal position flat footed on the bottom and leap straight up as fast as you can for 1-2 minutes also helps the core.
Just make sure your eating a few carbs about an hour before for workout energy and eating protein/greens within 30minutes after workout. You're muscles are worn and open for nutrients and looking for them to rebuild your muscle. If you don't your body will get those nutrients from wherever it can find them, stored fat, muscle, organs and you'll be fatigued frequently afterwards.