Valley of the Rogue Magi (16-20) - in Commonlands, requires access quest the first time, read the poster on the signpost at the crossroads. The zone itself is in the canyon beside Ruins of Valmarr. Medium sized zone with many possible named bosses.

The Shattered Vale (16-20) - in Antonica, to gain access, inspect the dead body guarded by wolves on the road north of Stormhold. Smaller instance but has a nice little "event" to complete.

The Prison of Admiral Grimgadget (20-25) - this is a fun event instance, with a series of puzzles to complete the first time. Access quest "999 Year Old Port" is obtained from Heron Cogcarrier at the docks in Commonlands, or Sparrow Cogcarrier at -2392.52, -38.89, -342.34 in Antonica.

A Pirate's Hidden Stash (20-23) - on the first beach in Thundering Steppes, look for a small bump on the beach that you can highlight. Medium sized, medium chance of named bosses.

The Rumbler Caves (22-26) - also in TS, at the sharp bend in the dry riverbed, just east of Thundermist Village. Again look for a bump that is highlightable, this one is very shallow and hard to spot. Medium sized, small chance of named monster at the end.

The Necromancer's Lair (25-29) - in Nek Forest, access quest is given by an NPC in the camp next to Behemoth Pond. Smaller zone, but has an event at the end with a boss encounter, and a guaranteed piece of loot.

A Gnoll Cave (25-29) - in TS, you must complete 2 quests for Tish in tower 3 to earn access. The zone in itself is just a few yards from tower 3. Medium sized instance with some scripted events and a boss elemental.

An Open Grave (27-30) - I never found this one myself, but I believe it is where the goliath skeletons live.

Tallon Hording Halls (29-33) - in Zek, in Blood Ore Valley, look for a door in the valley wall (it can appear in various places). Kill the 1-2 orc guards to enable entry. Small instance with small chance of 2 named bosses.

D'Morte Burial Chambers (30-34) - Bloodlines adventure pack. Located in Nek Forest just west of Gul'Thex citadel. Contains some heroic mobs that you do not have to kill.

Dire Hollow (31-35) - Bloodlines adventure pack. Talk to NPC in the camp at Timber Falls. The zone in itself is a little ways downstream on the river bank. Medium sized zone, not too interesting.

Sullon Mines (32-36) - probably the largest solo instance. In Zek, in Mythical Forest, look for a doorway around the valley wall guarded by 1-2 orcs (its exact location varies). Large cave system with at least 2 potential bosses.

The Cave of Wonder (32-36) - in Enchanted Lands, a very small bump in the ground (very hard to find) in the vicinity of the Bootstrutter marker in the spider area. Small instance, I never saw a boss. The monsters are all casters.

Cavern of Tangled Weeds (34-37) - also in EL, very small bump in the ground (very hard to find) in the twisted nymph area close to the lamias. Very small zone, don't expect much from this one.

Underrot Caverns (35-39) - Bloodlines. Talk to NPC in the camp at Timber Falls. The zone in itself is a little ways downstream on the river bank. Medium sized zone, more fun than Dire Hollow, with possible named bosses (roughly level 38).

Cleansing of the Cove (36-40) - in Feerrott, complete Mashara or Tarin's quests to earn access. One of the better and larger instances, and one of the few that respawn. Event at the end with a boss final fight, with guaranteed loot.

The Dark Den (37-40) - in Zek, look for a doorway guarded by scorpions in the valley wall around Titan's Tower (or even on the ramp up the tower), the exact location varies. Very small zone, but good chance of a named drake who drops loot.

Forbidden Sepulcher (39-43) - in Everfrost, inspect the dead barbarian in the "monitor lizard" valley. This instance is easier than Everfrost itself. Medium sized, fairly easy and fun, and has a boss event at the end, with guaranteed loot.

Splitpaw Arena (20-50) - Splitpaw. Splitpaw adventure pack. You must reach the Lower Tunnels village first. This is a trial of progressively harder opponents, 9 in all. You can quit at any point and collect rewards, but if you are defeated at any point you get no reward.

Delving into the Darkness (20-50) - in Thundering Steppes, the gnoll "terraporters" along Thundermist Road. Large solo instance with puzzles to solve using exploding barrels and stackable crates, and a boss at the end.

Trial of Harclave (20-50) - Splitpaw adventure pack. You must reach the Lower Tunnels village and then listen to the storyteller. A very unique and fun instance because you get a buff that lets you defeat heroic mobs. A long instance that takes up to 2 hours to finish.

The Hideout (20-50) - Splitpaw Adventure Pack. Get the quest "Flitch's Prank" from Flitch in the gnoll village (Lower Tunnels). A fun quest, with a boss at the end. Give yourself at least an hour to finish it solo, or bring friends.

Azhar's Penitence (47-52) - in Desert of Flames, in Sinking Sands, on the medium-sized island visible from the dock. Rub the genie bottle to gain access. You will face several opponents and then win your choice of reward.

Tears Grifters (45-60) - in Maj'Dul, talk to any Tears Grifter. Short missions that scale to your level, usually involve killing a handful of bad guys, and getting cash/XP/faction rewards. There are about 15 missions that are given out randomly, and you can request a new mission every 30 minutes.