From what I've seen, Hinduism isn't a religion like most of the other modern ones with many "rules" that you have to follow, it's more of a "pick-your-own-rules" way of doing it. (See: Is there an Orthodox Hinduism?)
Additionally, it changes much as time passes1
The question is, are there any invariant "core" tenets/rules to Hinduism; i.e. rules that have not changed since Vedic times and are still a part of the religion.
- The no-conversions rule (one must be born a Hindu). This seems to be changing now.
- Restrictions against beef
1. So do other religions, really.