If your safe isn't close to an outlet, I’d definitely recommend picking up a rechargeable desiccant canister, like the Hornady X-Large. It's super easy to keep an eye on, and when it needs a recharge, you can just pop it in the oven. I've actually used those in smaller safes before and they work like a charm.
Most people go with one of two routes: a GoldenRod-style electric rod if you’ve got power in the safe or rechargeable desiccant packs if you don’t. I run a rod plus a small digital hygrometer so I can actually see humidity levels instead of guessing.
Most long-term collectors use Goldenrod dehumidifier rods. They’re passive and don’t need much upkeep, providing steady low heat to keep moisture out without cycling on and off.