Meta: old thread, but one of the top google search results. so here my 5 cents.
https://www.duplo-schienen.de/lego-duplo-schienen-bauanleitungen.html German Website, has 40 setup plans (linked above) plus also some general rules when it comes to track design, track switches, crossings. Top rule: always connect a switch's single end to a single end of another switch, never connect the single end to one of the double ends - or some parts of the track might never be reached.
My "organic oval" design rule:
Create a semi circle (6 curves), add any number of straight tracks at any space in between (or at the end), create the exact same setup a 2nd time and you can always connect both to a closed track. examples here: https://ibb.co/ZKKCxFn
Some more ideas that work without tension in the track: https://ibb.co/SRmygj5