Some Cool Possibilities
Address Form Autocomplete
When a user begins typing the first few letters of a city, you present an autocomplete list of suggestions. Especially users on keyboard-constrained devices will appreciate (and probably demand) it. Implement this using the city name-prefix filter. Enhance by sorting by population, descending, to show larger cities first. The Find Cities API is your friend.
Only Show Large Cities
Maybe you're displaying a map of a country and you don't care about every little podunk town with a lone gas station and mom-and-pop pitstop eatery. If so, use the minimum-population filter to return only cities with whatever minimum number of people defines a city on your map. For added coolness factor, you can automatically lower this number, revealing smaller cities as the user zooms in. Again, the Find Cities API is all you need.
Only Show Nearby Cities & Towns
Your app helps people decide which neighborhood to move into. The user first selects from a list of major metropolitan areas (minimum-population filter). Once a selection is made, you display the regional list of smaller cities within a 50-mile radius of that metropolis. Implement this directly using the Find Cities Near City API.