Trove is a sandbox game where you can collect weapons and gear and level-up to fight monsters in a 3D environment. This game has lots to do and you'll probably be hooked once you start.

My skills with Blender are very limited but for my purposes, Blender has been great for creating models! I recommend this software for your model-creation needs.

Undertale is a game with a great story (I won't spoil) that you can play through many ways. The graphics, gameplay, and music are great and I completely recommend this game.

Terraria is a game where you start with a few basic tools in a 2D world and you can use these tools to collect much better items through exploring, crafting, and fighting for an enjoyable experience.

Scrap Mechanic is a game where you can engineer anything but only using barrings for movement. You would be surprised how much you can make, and at the same time, it is super fun!

Unturned is a zombie survival game where you choose to start in a number of worlds, and then you have to find items to rise to the top and survive the zombie onslaught. You can play with friends too.

Race The Sun is a game where you have to endlessly wind around obstacles and collect items to keep the sun up so your solar battery doesn't run out. This game is fun but it got old after a while. :/

Cubic Castles is a sandbox game where you can build pretty much anything. This sandbox game takes place in realms and everyone has their own space to work in that they can share with others.

Garry's Mod is an awesome sandbox game in my opinion. If you don't like what GMod has to offer, then you can download thousands of user-created addons to improve the game to your own tastes.