The theme I went for is heathland, all the rides have latin/scientific names for plants and animals that are (to me at least) typical for heathlands, I'll let you figure them out (a quick google search should do). The park name is "purple heathland" in latin.

It doesn't really have a story, but rather a rough timeline of when each part/building was built.
- around 1890, the barn at the entrance was built as a place to keep the sheep that grazed the heath to make sure no trees would grow
- around 1980, the carrousel and helicopter ride around the square
- around 1990 the lay-down coaster and conversion of the now disused barn into a food court
- around 2010 the hybrid coaster (quite ironic that that one ended up being the largest coaster since that contest was last month)

There's a colour palette with this as well.
I would advice you to turn on the "disable plants withering" cheat.
Thanks for the nice incentive to get creative!