Since there are three winter activities and each participant has an equal chance of receiving a coupon for any of them, the probability of receiving a coupon for sleigh rides is \( \frac{1}{3} \).
To find the expected number of participants who would receive a coupon for sleigh rides, you can multiply the total number of participants (900) by the probability of receiving a sleigh ride coupon:
\[ \text{Expected number} = 900 \times \frac{1}{3} = 300 \]
Therefore, the expected number of people who would receive a coupon for sleigh rides is 300.