I bonds have a 12 month minimum holding period. If you withdraw before they're 5 years old, there will be a 3 months worth of interest penalty. It's $10K per SSN per year. Interest is state and local tax free. If interest is used for higher education AND you make less than $83,200 MAGI (see IRS Form 8815; that's the number from 2021 for a single filer) then the interest is federal tax free too. If you make over $98,200 MAGI (single filer in 2021) then the interest is fully taxed at the federal level; the interest exclusion gets phased out between $83,200 and $98,200.
You'd think that a student could buy these to fund higher education, and get the interest exclusion, but no. If a person is under 24 years of age when the bonds are issued, then the interest is always taxable at the federal level...however if your kid files for themself and makes less than $12,550/yr (standard deduction for single filer in 2021) then they pay $0 federal taxes anyway, so functionally, it's not being taxed.
I bonds must be purchased from TreasuryDirect.gov
There is also the Ally bonus going on right now. See
https://www.disboards.com/threads/anyone-have-a-high-interest-savings-account.3898923/post-64365709