There are many investments which can be held in an RRSP, RESP and Tax Free Savings Accounts (TFSA - see link at bottom) including:
Money that is legal tender in Canada, and
Foreign currencies are qualified investments, subject to certain limitations. See Money and deposits of money in the Canada Revenue Agency (CRA) Folio S3-F10-C1 - Qualified Investments. Prior to March 31, 2006, foreign currency cash balances were not allowed under the bylaws and regulations of the Investment Dealers Association (IDA) of Canada. See the IDA bulletin #3522 from March 2006 regarding the amendments which now allow foreign currency cash balances in RRSPs.
Most brokerages now provide the option of holding foreign currencies in registered accounts. Brokerages make money every time foreign currencies are converted to Canadian funds, so there was not much incentive to provide this service, other than it makes their brokerage much more attractive to RRSP holders. To reduce the cost of foreign currency transactions in registered accounts if your brokerage does not provide the option of holding foreign currencies, see our article on washing trades.