What are a few time-consuming operations in C?
The Ackermann function is usually a fun one, but don't give it very large inputs if you want it to finish in your lifetime.
I know you said you wanted to make your own, but perhaps you could draw upon existing benchmarks for inspiration. The Computer language benchmark game has run many programming languages through a a set of benchmarks. Perhaps you can get some ideas looking at their benchmarks.
Some quick ideas of the top of my head:
Matrix multiplication: mulitplying 2large matrices is relativelycomputationally intensive, though youwill have to take caching into account
Generating prime numbers
Integer factorization
Numerical methods for solving ODEs -Runge-kutta for example