Classic Dungeonmaster

I think this is a nice way to make current players visit old dungeons and explore their forgotten secrets :).

There are some instances you can solo if you are 70 or 80, and for some others, you will need certain requirements.

These are the instances you can solo, and you don't need any item or additional player to summon a boss:

Deadmines - Just kill Edwin VanCleef
Wailing Caverns - Kill the four Fanglords (Lady Anacondra, Lord Cobrahn, Lord Pythas and Lord Serpentis), talk with the Tauren next to the entrance and follow him, then kill Mutanus the Devourer.
Blackfathom Deeps - Kill Aku'mai
Gnomeregan - Kill Mekgineer Thermaplugg
Shadowfang Keep - Defeat Archmage Arugal
Razorfen Kraul - Slain Charlga Razorflank
Razorfen Downs - Slain Amnennar the Coldbringer
Zul'Farrak - Kill Chief Ukorz Sandscalp, here's the explanation how to reach him:

In order to get to Ukorz and his trusty sidekick Ruuzlu, you must free the captives on top of the big temple. This triggers the temple event, where you fight a giant mob along with several NPCs. The goblin who lays mines and throws dynamite must survive. Talk to him after you're done, and he blows up the door that leads into Ukorz's area.

Maraudon - Defeat Princess Theradras
Blackrock Spire (Lower) - Kill Overlord Wyrmthalak - I did this one a long time ago, so I don't remember whether you need a key or something, sorry :(.
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Then the ones that are inside Stormwind and Orgrimmar:
Stormwind Stockade - Kill Bazil Thredd
Ragefire Chasm - Slain Taragaman the Hungerer
It may not be easy to achieve, but enter the city and do a corpse run to the instance portal. I'm Horde, and every time I died in SW I appeared at the graveyard in Eastvale Logging Camp, not the one near SW, so try not to die very much :P.
As I don't have an Alliance char, I don't know whether they appear at the graveyard near Orgri or the ones in Razor Hill/Sen'jin Village.
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I continue in the next comment.

- Deadmines
Location: Westfall, Eastern Kingdoms
Coords: 42, 71
Minimum Level: 10
Level Range: 16-22

- Wailing Caverns
Location: The Barrens, Kalimdor
Coords: 46, 36
Minimum Level: 10
Level Range: 16-25

- Blackfathom Deeps
Location: Ashenvale, Kalimdor
Coords: 14, 13
Minimum Level: 19
Level Range: 20-28

- Gnomeregan
Location: Dun Morogh, Eastern Kingdoms
Coords: 24, 39
Minimum Level: 20
Level Range: 24-32

- Razorfen Downs
Location: The Barrens, Kalimdor
Coords: 45, 89
Minimum Level: 25
Level Range: 33-42

- Uldaman
Location: Badlands, Eastern Kingdoms
Coords: 44, 12
Minimum Level: 30
Level Range: 36-42

- Maraudon
Location: Desolace, Kalimdor
Coords: 29, 62
Minimum Level: 30
Level Range: 42-51

- Blackrock Depths
Location: Searing Gorge/ Burning Steppes, Eastern Kingdoms
Coords: 34, 85
Minimum Level: 40
Level Range: 52-58

- King of Dire Maul
Location: Feralas, Kalimdor
Coords: 59, 45
Minimum Level: 45
Level Range: 55-60

- Stratholme
Location: Eastern Plaguelands, Eastern Kingdoms
Coords: 27, 12
Minimum Level: 45
Level Range: 58-60

- Ragefire Chasm
Location: Orgrimmar, Kalimdor
Coords: 52, 49
Minimum Level: 8
Level Range: 12-18

- Shadowfang Keep
Location: Silverpine Forest, Eastern Kingdoms
Coords: 44, 67
Minimum Level: 10
Level Range: 17-25

- Stormwind Stockade
Location: Stormwind, Eastern Kingdoms
Coords: 50, 66
Minimum Level: 15
Level Range: 21-27

- Razorfen Kraul
Location: The Barrens, Kalimdor
Coords: 43, 90
Minimum Level: 17
Level Range: 23-33

- Scarlet Monastery
Location: Tirisfal Glades, Eastern Kingdoms
Coords: 82, 33
Minimum Level: 20
Level Range: 28-42

- Zul'Farrak
Location: Tanaris, Kalimdor
Coords: 38, 20
Minimum Level: 35
Level Range: 42-48

- Sunken Temple
Location: Swamp of Sorrows, Eastern Kingdoms
Coords: 69, 54
Minimum Level: 35
Level Range: 46-53

- Lower Blackrock Spire
Location: Searing Gorge/ Burning Steppes, Eastern Kingdoms
Coords: 34, 85
Minimum Level: 45
Level Range: 54-60

- Scholomance
Location: Wester Plaguelands, Eastern Kingdoms
Coords: 69, 73
Minimum Level: 45
Level Range: 58-60

Hope this helps! Happy hunting!