Most of my characters tend to live in Cimerora. Solo grinding means working the Roman wall, or running Marcus Valerius missions, esp. on characters that rely on stealth. The logout bonuses you get there are more beneficial than most.
I have also taken to doing the Dark Astoria arcs on a number of my characters, mostly for the incarnate geemies and merits. But there's too much stacking mob debuff there so that stuff is solo only. Knives and Talons are fun to play against, though.
I tend to avoid Atlas Park as soon as I have outlevelled it. The zone is busy enough that characters logged out there take noticeably longer to log back in to.
I try to avoid Boomtown until I have well outlevelled it. But some of my favorite exploration badges are there.