I got distracted by the various minigames in Mii Plaza. Let's see... so far I've completed all of the puzzles, beaten Quest many times (both I and II) and unlocked everything in that, cleared 8 missions in Squad, conquered 8 countries in Battle (and just now beaten an army slightly over twice my size), and made my way up to floor 6 in Mansion.
And probably spent a frankly silly number of play coins on it all to achieve that. I think I went through over 200 during one boss battle in Quest II, by continually cycling characters through with the potion of valour. Still, I've not found many other uses for play coins, and I easily accumulate the daily cap of 10 coins just with the normal amount of walking I do (today's step count is at 5442, so that would be 54 coins if it wasn't for the cap).