If you haven't played it already, I think Harem Hotel is a great example of a sandbox done right. The creator doesn't consider it a sandbox because it does still follow a relatively linear story but there are a lot of things to do outside of the main progression. The story is also properly gated so that you don't stumble across disjointed interactions. You're building up a hotel, training the girls in various stats, exploring the city to find side content, earning money in various ways so you can unlock outfits, toys, and additional locations, etc. While there is that money grind for unlocking stuff, I never felt like it was egregious enough to hinder the gameplay.