I don't see how this helps. The combats are still going to be trivial if you rest-encounter-rest. To counter that problem in sandbox play, you need to constantly manufacture time crunches or geography constraints on rests. And those constraints will seem increasingly implausable and railroady - which is contrary to the essence of sandbox.
[MENTION=54380]shoak1[/MENTION] I am more than willing to admit that the DMG and modules don't give a ton of great advice for how to best handle attrition as a DM, which is why it's great that we have these boards to help find solutions! One thing that I've learned is that every day in the world need not be an "Adventuring Day" that pushes the PCs to their limits. ADs only need to occur when the players are attempting to accomplish something of significance in their world.
So in sandbox play, traveling between towns in a powerful kingdom need not be difficult (though for commoners, the threat of a single bandit raid along the road might have them travel in numbers with armed escorts). Making your way across the Desert of Despair to find the ancient lost White Pyramids and it's amazing treasure, which no one has ever returned from, is a whole different story. Now you have to ask yourself why no one has made it there and back before your party?
If you want the travel itself to be difficult, perhaps you have to pass through the territory of an evil race of sentient beings, who will relentlessly attack anyone who dares enter their territory. To stop and rest would mean certain death as you become surrounded. Or perhaps the desert is guarded by an adult or ancient blue dragon (depending on what level you want the party to be able to get through). This is a single encounter that would TPK a lower level party, but one that a fully rested one may be able to take on a few levels below the recommended.
I've mentioned this far earlier in this thread, but the lesson I've learned from playing 5e and tweaked to fit my players is that I run the game in two modes. Exploration Mode and Mission Mode.
Exploration Mode will rarely see the adventurers hit their daily XP limit. Players here are figuring out what they want their next mission to be. Will they try and get through the desert and plunder the treasure in the White Pyramid, or will they go investigate rumors of a new Kingpin running the city of Vorimport and try and recruit him as a spy? Either way, they will focus on the social and exploration pillars to gather information and make a plan to accomplish their chosen goal as we move into Mission Mode. Fights, when they happen, are designed to give the players a taste of the types of creatures and tactics they may be up against when they move to Mission Mode.
Mission Mode will have encounters far in excess of their recommended daily XP limit and consequences for abandoning the mission once they start it. They will need to apply what they learned during the Exploration phase to to avoid encounters and have a chance to complete their mission. Now, I can hear the complaint, "but constantly having time constraints is implausible." I'm sorry, but I just don't buy it. A living world reacts to the presence of powerful entities attempting to impose their will on it. There are either other intelligent entities working against them that can get the upper hand if they stop to cower, or the task is so difficult that no one else in this neck of the world has been powerful enough to accomplish it.
5E gives great flexibility so that DMs can tailor games to their players. It's exists to spark the imagination and serve as a starting point. It's easy out of the box so that new DMs don't accidentally kill off parties. Even the much vaunted 6-8 encounter per day guideline isn't a guideline, it's a red flag to new DMs that shouts "Hey Noob! I know you love our monsters but don't stick 20 encounters in a row in front of the players and expect them to live."
Could their be more advice in the DMG and Modules? Sure. But there isn't, so the best we can do is try and help each other with interesting ideas of how to make it work for our tables.
