The most important thing is to create a pure Java API - regardless of the REST, RMI, or any other protocol that you want to use. With a pure Java API, you can support any access method.
If you have no precedent for these other access methods, do not create them now. You can build it when you need it.
The easiest interface to add is the web interface, where your web application runs in the same JVM as your main API. I would do it if it works for your use case. Creating a separate console application that accesses your core API through the REST protocol (or something else) works a lot more.