I bond
A US savings bond whose rate combines a fixed rate with an inflation rate that resets every 6 months.
I bonds are bought electronically on TreasuryDirect, from $25 up to $10,000 per person per calendar year. They earn interest for up to 30 years and the rate can't fall below zero.
You can't cash one in its first 12 months. Cashing it before 5 years costs the last 3 months of interest. The interest is federally taxable but free of state and local income tax.
Example
With a fixed rate of 1.00% and semiannual inflation of 1.50%, the rate is 1.00% + (2 × 1.50%) + (1.00% × 1.50%) = about 4.02%.