The After Dark expansion for Cities: Skylines made changes that increased the land value of waterfront property, which is great for the coastal areas of your cities. Now you can do the same for the areas of your cities that don't happen to be near water, thanks to a collection of ploppable European-style canals. The Canal Blocks mod collection, created by modder CGVos, lets you access canals of various sizes and plunk them into your metropolitan areas.
In addition to being quite lovely looking, especially at night, these canals will increase the value of the buildings place alongside it, meaning those structures will have an easier time leveling up. You can find the new canal blocks in the Parks menu. Keep in mind that these canals do require upkeep (paid out of your Parks budget), have power requirements (there's a lot of lights at night), and contribute a small amount of sewage to your city's water grid.
On the plus side, you don't need to terraform or mess with water. They just plop right in like other parks. There are a number of sizes to choose from, so you'll have some flexibility when building them into crowded existing cities. There's a 16x4, 12x4, 8x4, and several 4x4 units to act as corners or turns. The 16x4 canal even has a little rowboat. It's adorable. There are plans for more in the future, including s-turns and a transition if you want to connect your canal to a real body of water.
Another nice thing: the land doesn't have to be entirely flat to plop in your new canals, and there's no increase in noise pollution, which seems especially useful for commercial areas that are close to residential zones. Mainly, though, they're just really nice to look at, add a bit of personality to your city, and are a nice and easy way to get nearby buildings to level up without doing any major construction.
You can subscribe to the Canal Blocks collection right here. If you do, anything else added to it in the future will automatically become accessible in your game. The collection currently includes a little outdoor restaurant and cobblestone plaza, and some nice pedestrian pathway bricks you can use to accent your new canal areas.