You'll be hard pressed to find a cheaper yet more effective series of workouts for generating fitness than simple Calisthenics. None of these require weights or specialized equipment, all of them will make you more flexible, stronger, and more fit both cardiovascularly as well as producing muscle tone and endurance. All of that translates into more energy for the day.
“The primary calisthenic exercises are:

• Sit-ups

Start with your back on the floor, knees bent, bottoms of feet
against the floor. Lift shoulders off the floor by tightening abdominal
muscles bringing your chest closer to your knees. Lower back to the
floor with a smooth movement. This trains your abdominal muscles.

• Crunches

Like the sit-up, except instead of bringing the whole torso area
closer to the knees, only a concentrated but shorter movement of the
abdominals is performed. Shoulder blades are lifted off the floor, and
abdominals tightened.

• Push-ups

Start face down on floor, palms against floor under shoulders, toes
curled up against floor. Push up with arms keeping a straight line from
head through toes. Lower again, to a few inches off floor and repeat.
You can have a partner put their closed fist on the floor under your
chest and lower your chest to their fist each time. Do not rest on the
floor or your partners fist when you descend. You should keep your head
tilted upward, your back straight. Do not rest on your shoulder blades,
even when you feel fatigue. This trains your chest, shoulder, and
tricep muscles.

• Pull-ups

Start by grabbing an overhead bar (called a chin-up bar) using a shoulder-width overhand (palms facing forward) grip. Keep your back straight throughout. Using your lat muscles,
pull yourself up to chin level (always with the bar in front of your
head) then slowly return to starting position in a slow controlled
manner. Avoid using the arms to pull yourself up and do not make jerky
movements to gain leverage. This primarily trains your lats or upper
back muscles, as well as the forearms. An underhand grip variation or chin-up trains both the back and biceps.

• Squats

Stand with feet shoulder width apart. Squat as far as possible
bringing your arms forward parallel to the floor. Return to standing
position. Repeat. If this is not challenging, you can do variations.
One variation is lifting one leg off the floor in front of you, putting
both arms in front of you for balance, and squatting. This is a
one-legged squat or pistol. Squats train the quadriceps, hamstrings, calves, and gluteals.

• Calf-raises

Stand on a platform with an edge where you can let the heels hang (e.g. a curb).
Use your heels to lift your body on the balls of your feet, then slowly
return to starting position. This trains the gastrocnemius. A seated
calf-raise trains the soleus.

• Dips

Jump up into position in between parallel bars or facing either
direction of trapezoid bars found in some gyms. Cross your feet with
either foot in front and lower yourself on the bars until your elbows
are in line with your shoulders. Push yourself up until you are fully
extended, but you do not have to lock your elbows. Press yourself up
and down for repetitions and you are doing dips. Dips focus primarily
on the chest, triceps, and deltoids.

” --Calisthenics - Wikipedia, the free encyclopedia

Over the last several Saturdays I've shared on some of these individually. Next Saturday I'll lay out my personal routine.