We have a particular fondness for sandbox games at Board's Eye View: games that let players follow any of several widely different routes to victory. For that reason, Xia (Far Off Games) is a perennial favourite. Most sandbox games tho' are competitive. It's a relative rarity to find a sandbox game that's fully cooperative and, likewise, playable as a solo game. Arise then Project EOS Rise from Random World Games: a space-themed game that uses worker placement mechanics. Players are in a race against time (ie: an end-game countdown) to reach refuge but they have wide choice over the paths they follow.
The premise of James K Baker's game is that there is a rapidly approaching intergalactic extinction event. Earth has had advance warning of this and the 1-4 players are the crew of a ship seeking a unique refuge planet. If you play with four players, you'll each take a role as Commander, Pilot, Engineer and Combat Specialist. You'll need to manage your own section, but you'll take collective decisions over what the ship should be doing. Play with fewer than four and you'll need to double up on some or all crew roles but it'll be quicker and easier to agree on actions. This means Project EOS Rise works well as a solitaire but we've especially enjoyed it with two players, each in charge of two crew roles, as that provides scope for debate without tying players up in excessive argument or leaving a dissenting player sidelined.
Tho' your ultimate objective is reaching the refuge planet before the intergalactic cataclysm, you'll want to train and advance your crew's skills, and upgrade your ship's capabilities, as these will aid you on your quest. You can hunt down FRBs ('Fast Radio Bursts'), you can visit locations to pick up valuable resources, and you can take on pick-up-and-deliver side missions for the benefits these will earn you. There are alien ships out there and you'll come into conflict with them but the extent to which you actively seek battle is up to you...
There's quite a lot to take in before you get to grips with Project EOS Rise but one of the beauties of any cooperative game is that you don't have to have every player fully cognisant of every rule before you start: players are always working together so they can help each other with omissions or gaffes. The rules actually suggest one player taking the lead as 'game facilitator'. It's a sensible approach in dealing with a relatively complex game but you do need to ensure that this player does actually 'facilitate' rather than taking over the game as alpha player and leaving the other players as mere spectator pawns. As with Xia, there's a lot of dice rolling (mostly custom six-sided dice), so a high luck and push-your-luck element, but that just adds to the adrenaline pumping tension of the game. And, be warned, Project EOS Rise is tough to beat: more often than not you'll face collective oblivion before you reach that safe haven. But if you do eventually think you've mastered the game, you can further ramp up the challenge...
Project EOS Rise offers ample replayability, not just through the multiple options for the players to take but because set up is varied for every play. Rather than the more common randomised tiles used in sandbox games, Project EOS Rise uses a conventional board to represent the universe but the population of that board is randomised by using a d20 and d6 to allocate everything to an assigned sector and galaxy. It may be initially time consuming but it works.