Well, for a Sandbox server, you will most likely need a HUGE MAP, a few tools to help you build and, of course, some fun addons to mess around with. I'll list a couple of suggestions below:
ULX & ULib, for administrating the server
3D2D Textscreens Advanced Duplicator 2, as godx said above --- this is a must, to save your constructions and easily dupe them at a later time AntiPlayerStuck Easy Animation Tool Easy Bodygroup Tool Extended Properties Keypad Tool More Materials PermaProps Precision Resizer SmartSnap Stacker Weight Wiremod (if you fancy complex sandboxing)
My 2 #1 addons (I can't decide which one is better than the other) is easily Five Night's at Freddy's 1 Map, and the Five Night's at Freddy's 2 Map, you can amp up the atmosphere in the maps even more if you get things like swep weapons, fnaf playermodels, fnaf npcs, fnaf entities and other things related to Five Night's at Freddy's!