I think this is a great idea which should definitely take off.

How about offering a cat feeding service for people who don't like putting their cats in catteries (believe me, you'd be surprised ).

Also, there seems to be a gap in the market for a spare key holding service, but that may be logistically impossible (ie. if someone can't find their key at 3am and calls you up!!).

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"Instead of looking for things that have gone wrong and trying to fix them...Look for things that went right and try to build on them" Tom Peters