Ever get that new perk or weapon or whatever and realize you gotta test it out against a bunch of baddies before you continue your quest? Preferably, you'll head someplace teeming with baddies, but without a lot of spaces to enter/exit. For example, my absolute favorite is Liar's Retreat: the very first room is just choked with Falmer. Halted Stream Camp and Silent Moons Camp are also good if you want to tangle with bandits and mages. You don't even have to go into the mines or interiors. The foresworn over at Karthspire camp and Lost Valley Redoubt are also quite fun.
All of these are easy to access, unlike boss fights which only occur at the end of a dungeon. I've got Dragonborn as well so any suggestions over at Solstheim are much appreciated.