Have you seen the fantastic game that is Ingress? If you haven't played, I would highly recommend it.
Ingress is an augmented reality game, meaning that the game landscape takes places in the real world. The core premise of Ingress is that two new technologies have been added to our world. The first is a huge number of Portals. These portals are located strategically around the earth at important sites. For example, one of the data sources used to establish the initial set of portals was the list of all post offices. Users can also submit landmarks or sculptures as candidates for new portals. I'll explain how portals work in a moment. The second technology added to our world is called XM. XM forms the basis for user power within the game and is consumed when performing actions.
The world is split into two teams, Resistance or Englightened and you choose your team when you start playing. The beliefs behind these groups is unimportant, as they basically equate to green versus blue. At any point in time, a given portal can be controlled or owned by only a single team. The game revolves around destroying enemy structures built on portals, and building your own.
To drive the game, there are four basic item types. Resonators, XMP Bursters (bombs), Portal Keys, and least important, intel. In order to collect items you must travel around in the real world (meatspace) in order to be within 50 meters of a portal. When you are actually that close to a portal, you are able to "Hack" the portal, which has a random chance of giving you items based on the owner of the portal, and the level of the portal. Hacking gives any of the four item types randomly.
Once you have a few items in your inventory, you can engage the main game mechanic. Use the online Ingress Intel Map in combination with your android-based local scanner to find unclaimed portals, to locate enemy portals and destroy their with Bursters. When you have destroyed a set of enemy structures of found an unclaimed portal, you can deploy resonators on it in order to capture it and power it up.
Once you have captured several portals and their associated keys, you can "Link" two portals together. Linking three portals into a triangle creates what we call a control zone, and wins your team overal points, at least until an enemy comes along and uses their own Bursters to destroy your resonators.
The game continues like this, with users gaining experience points for certain key actions, leveling up and gaining access to higher level items and portals. Higher level portals can make longer connections, but each user can only deploy a limited number of high level resonators to a given portal. To achieve a level 8 portal (which would require 8 level 8 resonators), you would need 8 different players on the same team to upgrade the portal and its resonators. Such a portal would be able to connect to other portals hundreds of miles away. Users need to be careful when they do this because although they may make a giant control zone, links can't cross each other, so a long link may prevent a lot of other portals from being linked.
As for being arrested, some of the time the portals are located in places like police stations, fire departments, cultural monuments. Driving and walking to these places and standing around in the middle of the night "Hacking" on your phone is not well understood by the authorities, and there have been several cases where suspicious behavior has resulted in an arrest. Many authorities don't understand the game, and treat anything they don't understand as hostile.
Unfortunately to play you need a recent Android device (I would recommend a Nexus 4 if you don't already have one, my Galaxy Nexus is a little bit too slow) and an invite code. Invite codes are given out somewhat randomly at ingress.com, and in various other places on the internet.