Lots of places to go...
Zones
Trick or Treating only works if the player is in a zone with a level appropriate for their current combat level. The following are the appropriate levels for various zones:
Heroes:
01 - 11 Atlas Park
01 - 11 Galaxy City
05 - 15 Kings Row
05 - 15 The Hollows
07 - 15 Perez Park
10 - 20 Steel Canyon
10 - 20 Skyway City
10 - 20 Faultline
11 - 20 Boomtown
20 - 30 Independence Port
20 - 30 Talos Island
20 - 30 Striga Isle
20 - 30 Terra Volta
21 - 30 Dark Astoria
20 - 30 Croatoa
30 - 40 Brickstown
30 - 40 Founders' Falls
30 - 40 Crey's Folly
33 - 40 Eden
40 - 50 Peregrine Island
40 - 50 Firebase Zulu
Villains:
01 - 10 Mercy Island
05 - 15 Port Oakes
10 - 20 Cap au Diable
20 - 30 Sharkhead Isle
25 - 40 Nerva Archipelago
30 - 40 St. Martial
40 - 50 Grandville
Praetorians:
01 - 08 Nova Praetoria
01 - 08 Underground Nova
09 - 15 Imperial City
09 - 15 Underground Imperial
16 - 20 Neutropolis
16 - 20 Underground Neutropolis
20 - 29 First Ward
30 - 39 Night Ward
All:
Since all PvP zones automatically exemplar anyone entering to a specific level, anyone entering the zone is brought to a combat level appropriate to the zone, so all zones are appropriate for Trick or Treating for anyone of a high enough level to enter the zone.
20 - 50 Siren's Call
30 - 50 Warburg
15 - 50 Bloody Bay (Bugged. Trick or Treat range appears to be levels 10-20, but since all characters are set to level 25, they no longer meet the level requirement.)
35 - 50 Rikti War Zone
35 - 50 Cimerora
40 - 50 Kallisti Wharf