When one gets the urge to get off the couch and hit the road to do a bicycle ride (or a unicycle ride, if you are like me!), having a bunch of ride routes planned out before-hand is a great thing. When struck by inspiration, you really should just be able to hit the road and not have to waste time wondering where to go.
Planning takes effort
Those of us who have the riding bug spend more time that we'd really like to admit, scouting potential routes, and this is a great thing. What's even better is if we remember to make notes from our rides, to note what went right, and more importantly, if something went wrong. These notes help us to replace segments with better segments. Is the traffic too bad, or the road quality pathetic, or both? Just note the section and remember to scout for a replacement. I also note the difficulty levels and the time taken, as this is very specific to just me. While routes made by others might be available on various platforms, my own personal notes allows me to personalize it with information about conditions, time taken, potential rest and photography spots and the like.
Plan for time vs distance
There are times when I just want to go on an exploratory ride, but other times, I want to do something more specific. I might want to do an easy recovery ride, but still get some saddle time during which I can get some light cardio. If so, I'll have a target distance in mind, but for those of us who have families, we hardly can claim the freedom of disappearing without any limits on when we ought to return, so plan for not only the distance but also the time taken. Note the time taken to do a relaxed ride on the route as that helps to setup a maximal time baseline. If you push harder, you know you can chop some time off that high watermark.
Since I regularly ride unicycles, and my speeds on a unicycle are much lower than my speeds on a bicycle, it's extra important to make notes of the time taken. This might be important to you too, if you ride different kinds of bicycles. A roadbike is faster than a MTB, the heartrate load is harder while slogging on a heavy MTB, and this means your time to complete a course can be significantly different depending on what you are riding.
I make courses of lengths that start from 10 km/1hour and then increase both the time and distance, so when I want to go for a ride, I just need to know how much time I have at my disposable, and the amount of distance I want to ride, and can just select a route that suits the occasion, instead of wasting time planning the route.
What tool/site to use to create routes?
This is all about personal preference. A good site/tool would have a user-friendly interface that allows you to quickly create a custom route, by searching for places by name, address, coordinates, or simply by scrolling across a map. A strong bonus is if the site allows you to save your routes and even export it in popular formats such as GPX/TCX which can be used in devices such as a bike computer capable of navigation, such as the Garmin Edge or Wahoo Elemnt devices, or a dedicated GPS device such as the Garmin Etrex or Garmin GPSMap, which allow you to upload both basemaps and routes. One site that's delivered on all of the above for me is komoot.com
Multisport features
Komoot allows you to specify the sport, allowing you to choose from road biking, mountainbiking, and trekking. Depending on your choice, it decides the best routing option, if you just specify the start point and the destination. It gets information about the surface condition and surface type from other users who've used the service. You can flag sections as bad add pictures and 'highlights', a short description of what's interesting/nice about a place.

The interface allows you to choose the sport
How to create a route
Register for a free account on komoot.com, login, and then head to komoot.com/plan to open the route planning page. You don't need an account to create a route, but you'll need one to be able to save a route, or add pictures or notes. If your start-point and destination are unique, you can specify them and it'll give you a route on its own, and you can then proceed to add more waypoints along the way, and you can choose strategic waypoints to force the site to choose a route segment that you'd like to ride instead of the segment chosen by the site. If the start-point and destination are both the same, just note the same place for both start and destination, and add waypoints that you'd like to cover on the ride, and it'll make the changes accordingly. Again, you can alter the chosen route as per your likes, by choosing waypoints off the selected course.

Start by specifying the Start and Destination points, select if it's one-way or return, and get started!