You want to gather as many stars as you can, since they're the in-game currency and also allow you to score more points. However, don't focus on them so much that you stop paying attention to your surroundings. Plan courses that will allow you to pass over stars, but only when you can do so safely. The same is true of keys, which allow you to open mystery boxes. You won't find as many keys as you will stars, though, so make sure you prioritize for keys when the opportunity arises.
Character Unlocks
Initially, every five stages you completes will grant you another character. However, the number of stages you must clear to access new characters will go up over time. You can also find characters in in mystery boxes, which you can open with keys.
Extra Spins (Exploit)
Once you spin and win a prize at the spin-to-win game, you can set the time forward on your phone (in Settings) to immediately qualify for another spin. You can repeat this as often as you like for all the free spins you want. It's a great way to build up a bunch of stars and keys, as well as various other prizes.