Roblox is a free multiplayer sandbox game.
You can visit others places and even create your own place and see people visiting it.
Obstacle course, Battlefield, RPG place etc. The choice is yours.
Roblox does not cost anything, but you can join the "Builders Club" to get more privelges, like having 10 places at a time, get a daily income of 15 robux and you even get a small "Builder's hard hat" :)