I bond

An I bond is a US savings bond whose rate combines a fixed rate for life with an inflation rate reset every six months, so it keeps pace with inflation; you can buy up to $10,000 a year.

I bonds can't be cashed in for 12 months, and cashing in before five years costs the last three months' interest. Interest is taxed federally but not by states, and can be deferred until you cash in.