I've mentioned this on The Tavern's YouTube channel, but the best campaign I ever ran was using the RuneQuest 2e rules with the Pavis and Big Rubble boxed sets over 30 years ago. We never called it a sandbox, but it was by far an open-world-style campaign and very driven by player actions.
I recently picked up the RuneQuest Starter Box directly from Chaosium for 30 bucks, and the value is amazing. I'm sure you could snag a copy on Amazon, but the instant gratification you get with the accompanying PDFs for purchasing direct is hard to beat.
Want to take the RuneQuest rules out for a test drive before plunking down a few bucks? Snag the free RuneQuest Quickstart rules.
A mythic world of mortals and gods, myths and cults, monsters and heroes. In Glorantha, the Runes permeate everything, and mastery of Runes allows astonishing feats of bravery and magic.
Glorantha is ancient, and has known many ages, but now it is at the brink of the greatest conflict it has ever known… the Hero Wars.
Glorantha is the setting of RuneQuest, one of the oldest and most influential roleplaying games ever published.
Welcome to Chaosium’s RuneQuest Quickstart Rules, a booklet that collects the essential rules for RuneQuest: Roleplaying in Glorantha and presents them in abbreviated form. Use this booklet to play the new edition of RuneQuest immediately, and to discover the improvements to the system. These game rules and a new adventure—The Broken Tower—preview the new edition of RuneQuest, developed in close consultation with the original designers and creators of RuneQuest and of Glorantha.
The Broken Tower, set in the heart of the Dragon Pass, is suitable for up to 5 adventurers and one gamemaster, complete with all the rules needed to play.
I'll likely record an unboxing video of the RuneQuest Starter Box on the Tenkar's Tavern YouTube Channel this weekend.