squats & push-ups are the basics
then
Hindu squats and
Hindu push-ups
After that sit ups are good for your stamina and toughening your abs.
The best way to do them is link legs with a mate. On every even rise (2,4,6 etc.) one of you throws a jab cross into your partners hands
on the odd rises (1,3,5) etc. swap punching over.
Do this for 100 rises
Then depending on your kit, depends on what else you can do.
I use a large oak tree to do 100 max's (I have no idea why it's called that)
This involves doing a technique in 2's, then 4's, then 6's etc. up to 10's, then back down.
2,4,6,8,10,8,6,4,2 = 50
you then do these 10 times, so you do 500 repetitions.
So that could be 2 punches, then 4 etc, or 2 leg kicks, then 4 etc.
Skipping is good at working your timings, footwork & strengthening your calves
It all depends on your body's limits, and the limits of your surroundings.