If you want to leave a harness on all day I would use a fabric "harness vest" - one with velcro would probably be safest.
A step-in like Cindy's would be easy to take on and off, but you probably don't want to leave that on all day just because of the hardware on it.
Here are a ton of harness options you can take a look at:
Harnesses for Small Dogs from Golly Gear