Some random discrete ones:
1. Prove there are infinitely many primes
2. Prove is a = b (mod m) then gcd(a,m) = gcd(b,m) (note = means congruent here in the mod case)
3. Prove that if n is prime, then for all integers a and b, if n|ab then n|a or n|b
4. If a and n are integers, prove an-1 is...