BTD 6 also has a bunch of pretty cool things you can find in the game. I'll explain all the ones I find the most intresting here and update it everytime there's something else added that I also find intresting.
There's a lot of cool settings for things like your towers and Bloons that you can unlock by getting achivements. They can range from pretty easy to ridiculously hard, so be prepared!
There are also a lot of little and big things you can find in the maps. Some examples of these would be things like the egg behind the waterfall on Quarry, or the flowers on Lotus Island. Others I won't spoil though, so go look for them yourself.
Speaking of little thingies, there are a lot of little fun animations on places like the title screen you can find by clicking on them.
that's it, they're just cool.