Hi Ellie,
I wish this was a simple answer. An RDSP is no different than an RRSP in that it can hold a wide variety of investment products such as GICs, bonds, stocks, ETFs, Mutual Funds, etc.
Unfortunately, your ability to purchase these products will be dependent on where you hold your account. Most banks don't allow you to purchase ETFs through them as they trade on an exchange. If you use the self-directed investing approach through their brokerage platforms, then its a possibility. That is, IF you can open an RDSP account. Not all discount brokerages allow it.
My advice would be to reach out to your discount brokerage to see if you can open an RDSP account which would then allow the purchase of ETFs. Legally it is allowed, but you need to find a brokerage that will allow you to do it. That might involve calling around to the Canadian banks/brokerages and letting them know your intentions.
Mat