Whether you need it or not, you have to withdraw the required minimum amount from your RRIF. However, nothing's stopping you from saving outside your RRIF once you've made the mandatory withdrawals.
You can invest the excess portion of your post-tax withdrawal in a TFSA (Tax-Free Savings Account) up to your contribution limit. Investment income earned in your TFSA is tax-free, and so are withdrawals. Plus, there's no age limit for contributing.
TFSA withdrawals also don't affect income-tested benefits or credits like the Age Credit, Old Age Security benefits or the Guaranteed Income Supplement.