pokemongolive.com
This one might sound like an odd suggestion if you don’t already know what Pokémon GO is, but please hear me out! Pokémon GO is a game that is designed to get you outside exploring the city around you. Pokémon are little monster characters that inhabit the world outside your window, and by walking up to them you get the chance to catch them and add them to your collection. There are a lot of other features like battling them, or using your strongest Pokémon to defeat big boss Pokémon, but you can take part in this aspect of the game as little or as much as you like.
Exploring different areas allows you to find different types of Pokémon. For example, if you live in a city then you will tend to encounter different Pokémon to those you might find at the beach. You want to catch some water type Pokémon? Head to a nearby river! You want a grass type Pokémon? Then get to your nearest park!
I played this for a few years, mostly on my walk to and from work, as well as on my lunch break. It got me walking more than ever, and I even met other players doing the same thing.