I have mine invested in some sort of mutual fund. I know, I know, mutual funds are the devil. However, I've also found that the investment options for RESPs are a bit more limited than other types of investment accounts. Someone please correct me if I'm wrong.
For me, the biggest value is in getting the most grant $ possible from the government. I believe it's a max of $500/yr, provided you contribute $2500/yr. That's a better return than I'm likely to see with anything else.
I believe the particular account I have also rebalances over time from more aggressive to less aggressive. So you're less subject to market corrections as the child approaches university age. This is a common feature of RESP funds.