Terraria (GOG) has been a family favorite for a few years now. The four of us will play co-op via LAN for hours each week and eventually get tired of it... then a few months later we'll create a new world and start playing a few hours a week again.
This game is never ending. You can explore, build, craft and fight all kinds of creatures forever. There are costumes, special events, bosses and all kinds of biomes. There is never a dull moment as long as you enjoy digging. You will spend hundreds of hours digging for ores and crafting them into bars to build your weapons, tools, armor and much more.
Terraria has re-invented the platform game with its huge sandbox worlds and non-linear exploration. Go anywhere you want, avoid traps and hordes... store all of your loot and go looking for more. What else could you want?
Quests?
Yes, Terraria has that too...
Tower Defense?
Yes, Terraria has that too...
Trading?
Yes...