I beg to differ. I have been running Traveller sand boxes for a couple decades and the system has no XP at all. The trick to worrying about only the mission being interesting is actually being interested in the adventure in the first place. I fully understand for some players they just want an avatar to move around and do things, but others expect a bit more out of their game. A good meta-goal goes a long way towards keeping a sandbox productive as most things, even those not mission related, can be used to move the game along.