Are you still playing Cities: Skylines? Personally, the only city I'm visiting these days is Los Santos in GTA 5, but the nice thing about mods is that they can pull you back into a game you haven't played in a while. The CityCopter mod, created by modder InconsolableCellist and currently in alpha, is a good example. It gives you a helicopter hangar and landing pad, and lets you hop into a chopper's cockpit and fly around your city in first-person.
There are no real goals, it's just kind of enjoyable to fly around a city you've built from the ground up instead of one someone else made. You can entertain yourself by checking out your city from some new vantage points, by trying to land on chopper pads on top of buildings, by threading the needle through your city's arch, and by trying to swoop around and not collide with things (though there's no penalty for doing so).
With the mod installed and activated, you'll first want to place your chopper pad, which appears on the unique level 3 building menu. You'll also see a new settings icon, in green, in the top right corner of your screen. Here you can change the hotkey to climb into your chopper (by default, it's Ctrl+G), and tweak other settings, like field of view and whether or not you want building and ground collisions on or off. Your mouse controls your pitch and yaw, and WASD controls the throttle and roll. Holding shift while moving the mouse lets you look around the cockpit without changing direction.
You can also activate night mode, which blankets your city in darkness, along with a chopper spotlight, so you can freak out people on the ground by shining a light on them. You can also turn off height limitations and look at your city from far, far above.
In its current state the controls are a little awkward, but with some practice I got pretty good at piloting my helicopter around. I'm looking forward to seeing where the mod ultimately goes.
You can subscribe to CityCopter on the Steam Workshop. There is also a donation link on that page if you'd like to contribute to the modder.