I ran into this problem on the first couple of layouts I built. Once the layout was running, I sorta lost interest in working on it after watching the circle a few times.
Then I think when I had my 3rd or 4th layout which included a loop for continuous running, I read an article in a magazine about "operations" The article described drilling small holes in the roofs of cars and using colored thumbtacks with numbers corresponding to industries to route cars.
I didn't want to drill holes in my cars, but the idea stuck with me. I came up with a similar system. I bought some plastruct girder channel, with the channels wider than a model roofwalk. I cut this 1 inch lengths. On one side I put the name of an industry on my layout, and on the other side either PRR or C&O which I decided I would interchange with. I designated two sidings a ways apart on my layout as interchange tracks for these RRs respectively. Each would hold 3-4 cars. I had some other sidings and about 6-8 industries on the layout including freight stations and team tracks (take any type car). I had about 12-15 cars on the layout. I placed cars at the industries and on the interchange track.
Some of the industries took only boxcars, one tanks etc. The team track took any kind of car and the freight station gons and boxes. So I made up 8-10 tags for each of car. I placed them in small boxes on a high shelf so I could see into them.
When the cars were on the interchange tracks, I would remove the tags from their roofs and put them into the appropriate boxes. Then I would randomly grab tags for each car on the interchanges from the box for that car type and place the tag on the roof walk or top with the industry name facing up. When the cars were at the industries and all switching was done, I would turn the tags over on the industry cars thus routing them to one or the other interchange.
To operate, I would select a loco from engine house (from the 3-4 I had), and a caboose. The engine house was near most of the industries. I would gather up the cars from the industries and then I would run around the layout to the first interchange track. I would pick up the cars from that interchange, and then set out the ones I had going there. I would run to the second interchange and do the same. Now my train had all cars routed to industries. I would run around to where the industries were located and set out the cars to whichever industry they were routed to. Then I would take the loco back to the engine house and shut down. With 12-15 cars, 2 interchanges, and 6-8 industries, that operation would take me from 1/2 to one hour. My layout did have a loop, but the part I ran was a spiral from the interchanges on a base level to the industrial area on the upper level.
Once I started doing this, the boredom disappeared, and I got very interested in running the layout this way and working on it. The system was simple, easy to set up, and generally each session was different because of randomly drawing the tags, and finding the cars in different spots.
I have moved on to use car cards and way bills and now I use a computer program to route the cars. My layout is much larger, but concept is the same. And it is always fun to operate and has been for years.
