Capital letters which read the same normally and upside down are: H, I, N, O, S, X, and Z.
Words that can be made from those to satisfy your criteria might include:
NOON
NON
SIS (slang)
SOS (acronym)
Palindromes
noon
race car
Racecar
racecar
A word that is the same spelled backwards and forwards is called a palindrome.
Camac street
A circle or a square.
If you spell NOON in capital it is spelt the same upside down and backwards.
Dad Pop Abba
NOON
I