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I'm okay in either case, close burst 3 or ranged 3 works for me. Probably better as a ranged 3, but I don't remember seeing any ranged powers that targeted anything but creatures. Only close and area effects have seemed to target spaces and non-creatures.
Noxious Grenade is an Area burst 1 within 10. While it IS an area attack, it isn't a close one. The caltrops would just be a burst 0 within 3.
Cloud of daggers is a better example. While it does have a normal attack, it's a ranged attack that targets a space and non-creatures. You can use it JUST to make the zone and it's ranged.