IMO after watching mine tear around the 20 long for the last little while, a decent sized harlequin rasbora needs 24" at least for swimming...they are pretty active. But, might work.
Blue ram or pair might work. They can be pretty stunning fish when in good condition. But I think a lot comes down to the actual tank dimensions. I wouldn't keep a breeding pair in smaller than a 24x12 footprint for a long time. Not quite as nippy as apisto pairs to each other, but still need some space. If you go single ram get a male...but he may not get the best coloration without a female to impress.
What about trying brackish and go with some less common fish like bumblebee gobies? You could probably do a decent group of them in a tank that size because they max around an inch. I have 3 in a 10g and there would be plenty more room if I didn't also have a puffer. My guess is your wife might think the gobies are cute and/or interesting. My wife does with the ones I picked up the other day.
