There are limits on what I bonds you can purchase, which is now $10K per SSN per year, and today with fixed rate of 0% the actual return for the maximum $10K per SSN is around 8.4% roughly, minus a 3 month penalty if you cash in before 5 years.
BUT that would only be if you purchased the I bonds by the end of April. This six month period, starting on May 1, pays 9.62% but what the next six month period pays is dependent on what inflation comes in at...unknown for a while.
So anyway, I bonds do not work for large purchases...fortunately we bought ours when the max per SSN was $30K per year.
If anyone is interested, you can find out more at the Treasury Direct web site:
www.treasurydirect.gov