You’ll get plenty of recs about doing a government wipe
You’ll get plenty of recommendations for various acts of violence which might or might not make any difference.
The thing is, there’s “actually safe” and “safe enough.” Drilling through the platters or whatever will leave quite a lot of data intact, which is potentially recoverable. However, such recovery is also very expensive, so for practical purposes you’re unlikely to be the target of such an attack unless you’ve got valuable corporate data on there.
The easiest way would probably be to run a factory restore on it. This will hose all your data and set it back up as it was when new. This could leave data behind which is recoverable by forensic methods, but really is safe for all practical purposes. Depending on the age, someone might then find a use for them, charity shop maybe?
The best way is DBAN. Copy it to CD, boot off it, let it do its thing.