score:7
Don’t think any RER trains are actually included (the Interrail site explicitly says they are not included), however some stations are served by both RER or Transilien services and TER services from other regions.
So you can go to Dourdan, Creil, Mantes-la-Jolie, Versailles-Chantiers for instance, you just need to make sure you take the right train. They may depart from a different station in Paris.
The Interrail route planner will tell you which trains can be used or not on such a route.
One advantage is that those trains will usually be quicker as they have much less stops. The drawback is that there are many routes which are excluded. Not sure if there’s an easy way to find all the relevant routes.
I found these:
In all cases, you need to use the TER service from the other region, not the RER or Transilien.
As you may see, most are at the outer reaches of the RER or Transilien networks, sometimes actually outside the Île-de-France region (but served by Île-de-France trains). Some have very few trains each day, others many more. Most are free with an Interrail pass, a few (towards Normandie) require a seat reservation.
Bonus: you can also travel between CDG airport (RER B), Marne-la-Vallée Chessy aka Eurodisney (RER A) and Massy Palaiseau (RER B) by TGV, without going through Paris. The trip between the first two is probably the shortest TGV trip you can make (9 or 10 minutes), and it runs on high-speed track. All those require a seat reservation. There’s also a Massy-Palaiseau - Versailles Chantiers - Mantes la Jolie by TGV (but on classic track, and only once a day — also note this one actually stops at the Massy Palaiseau RER C station while the others stop at the Massy TGV underground high-speed station).
Note that in many cases when a seat reservation is required, it might be more expensive than a regular ticket for the same trip!