Coins are the most common reward, but the other two show up fairly frequently (approximately 5% chance of each). Because of this, a good strategy is to let the game run while you do other things (like homework), but do a quick check every 15 minutes or so to clear the Snow Piles - you will eventually get a few silver coins to play KFS with. The most Snow Piles that will accumulate in your village at any one time is around 7 or 8, regardless of the amount of time you leave the game running or how much empty land you have. So remember to clear them periodically so more can show up!
Some Missions involve clearing a certain number of Snow Piles.
- ↑ Based on 450 trials which resulted in 20 acorns and 21 silver coins. This gives an value of 4.4% and 4.7% ± 1.9%