After finishing off 3-starring the 100cc of all the cups in Grand Prix, I've been steadily knocking out the 150cc ones.
On top of that, I believe I'm about halfway through getting all the coin-based unlocks. Mostly. My quick searches say at about 3100 coins you'll have everything not Gold Mario-related unlocked, and I'm at about the 1500-1600 coins collected range. So I'm getting there, which is nice.
What's also nice is I think I've made my Informed Kart Decision for a nice-looking load-out that also has some pretty good stats (including the hidden Mini Turbo and Invincibility stats, which is... a Decision to Have). It's not really required, of course... you can do just fine with whatever load-out feels right to you in Karts.
I'm just the sort that likes to look into these things so that my choices have some Good Number included in them for safety's sake. At the very least it's not yet another Yoshi in a Teddy Buggy with Rollers...