There are several "playable" outside races which could fit right in. Check out the Manual of the Planes and Planar Handbook. Spikers, Shadowswyfts, Neraphim, Bariurs, Aasimars, Tieflings, and other planetouched (Genasi of Faerun, those from Fiend Folio, etc) can all be fairly low-level. Even things like Githyanki and Githzerai, Azers, etc could work well. Then you can just put weird and wacky folk there too. Araneae, Barghests, Centaurs, young Dragons, Blink Dogs, Doppelgangers, Hags, Harpies and Hell Hounds, Ogres, weirdo magical beasts like Krenshars - or how about a Sphinx? - odd outsiders like Mephits and Magmin, or Salamanders, or even just playable races from other settings (e.g. Warforged/Changelings/Shifters). These are all CR5 and lower, and perfectly acceptable in somewhere like Sigil. Most of those are MM, too; if you crack open some of the other books, I'm sure you can find even more unusual stuff.