To unlock everything, you must:
Unlock all characters. To do this, you must 5 Star the Grand Finale Challenge, which will unlock Eliot and net you this:
To unlock the Grand Finale challenge, you must earn 4 stars on all other challenges in the game.You can also unlock him by hitting Rank 1, as detailed below
Unlock all outfits for all human Characters. To do this, earn 25 stars with each character, which will unlock this:
To unlock Eliots costume (and Eliot if you don't have him by this point), you must hit Rank 1 - Living Legend, which will also earn you this:
To speed up ranking up, I recommend 'Come On, Ride It', which can grab you 600k points and is straight forward on Hard. Playing on Hard is significantly higher scoring than Easy and Medium, so playing on Hard will definitely speed up the process.
That is everything. Chances are, the Living Legend and Got This on the Unlock achievements will pop at the same time since all the other requirements can be done quite early on, but to hit max rank the game requires a lot of playing.
Hope this helps!