PvE

That'd be up to you really. What I consider fun may not be fun to you and vice versa. All I can suggest is to try some stuff out. Hit Ouroboros and run some arcs, run some SF's, or explore the non-farm AE missions. You're more likely to find something you consider fun by actually searching for yourself, rather than asking here.

If you want high-level stufz, I've found all of the Patron arcs to be entertaining, as are the arcs from Television, Efficiency Expert Pither, and the Vanguard contacts in RWZ. I'm also a big fan of any TF/SF, whether it be blue, red, or co-op. I think there's a lot of re-playability in them but YMMV.

I encourage you to pony up the $30 for GR though so you can take your perma-dom through arcs by Maria Jenkins, Unai Kemen, and Crimson for more "endgame" content. Don't forget AE too, some players have come up with pretty neat stories and mission sets.

Then there's always the popular "Roll a new character and do it all over!" choice. If that doesn't fit your fancy though, maybe you should go back to farming pillboxes.