Well, the “sandbox” bug is going around the gaming community and it’s bitten me as well. I’ve decided to see if I can run and enjoy a sandbox game for a while.  And, as sandbox games are a very “old-school gaming” idea (like “1978 blue box old-school”), I have implemented some house rules to underscore this aspect of it.
First and most obvious, is to use the “slow progression” XP chart. Leveling back in the old days took forever, and a new level was a big deal when it finally hit.
Second, and related to the first, is to institute Class Training. Once upon a time, you didn’t hoard gold in order to purchase magic items, you had to spend it in order to go up a level. As I recall, none of the games I was ever involved in actually used that rule, because you had to earn the XP and spend the money for training, which seemed like paying for the same ground twice. But from a game-design point of view it does have some interesting wrinkles, not the least of which is that giant gems behind obvious deathtraps suddenly become enticing, instead of something that the players shrug at and move on as not being worth the time expenditure. (“Let’s go find some monsters to crack open for XP, shall we?”)
In order to avoid the “paying for the same ground twice” feeling, my house rule is very simple: between adventures the players can “trade in” treasure, at the rate of 1 GP = 1 XP, on “training.” Whether that training was actual drilling at the martial arts academy, studying tomes in the library, or blowing it all on ale and wenches, doesn’t really matter. The point is that treasure is now actually useful for something besides just buying another +1 for your battleaxe.
Third, and this one is a little more out there, is that a character may not progress past 10th level in any single class. At 11th level, should the game reach that point, they can multiclass, or they can take prestige classes as desired, but 10 is the cap for every class. This means that a lot of upper-end class abilities are just not available, and unless you are in a prestige class that gives the “+1 spellcasting level” there is no way to ever get a 6th-level or higher spell. It also mucks around with spell penetration at higher levels, so if that becomes a problem I’ll probably either nerf critter SR or provide some other way around it, but we’ll get to that later.
The next step, for me, is to create a bunch of hooks and/or adventure sites ranging from levels 1-5. Back when the model was “the DM has a signature dungeon,” you’d pretty much create a single dungeon where the first level had critters with 1-2 hit dice, the second level had critters with 2-4 hit dice, and so on downward, and the players would either fight, sneak, or negotiate their way through going up or down … and the GM would repopulate the place periodically to account for the new vacancies in dungeon population created by rampaging PCs. I have in mind to get around this by having a small handful of thematic “dungeons” where most of the action will take place, and the PCs can bounce around from one to another as they see fit. As they “get to know” these dungeons I’ll add new locations and probably toss in the occasional “big event” adventure for variety.
The key to it all, however, and something I’ll need to make sure to impress on the players, is that they’ll be the ones driving the campaign plot, in as much as there is one. As such, I may institute something similar to the “goal” mechanic from Ghostbusters: each character has a self-defined but explicit and measurable goal, and when they achieve something related to that goal, they get a bonus reward (probably in the form of XP). For example, a wizard might have the goal of “study magic phenomena,” and whenever they encounter something weird (like a magic fountain randomly tucked into a dungeon room for no apparent reason) they get an XP bonus for figuring it out. A fighter might have “defeat notable foes” and get bonus XP for fighting bosses, that kind of thing. Knowing the players’ specific goals can also give me ideas for encounters to throw at them. If the paladin has “slay demons” as their goal, I know I need to put demons into the game; if the rogue has “pick pockets” as their goal, I need to put them into situations where pocket-picking is feasible. That kind of thing.
I’m hoping this will be fun and a bit different. I’m also hoping that this will get me back in my GMing groove again for a while, ’cause I always miss gaming when I don’t get enough.
 As for exactly what that entails, the definition is pretty vague. But my interpretation is mainly that instead of coming up with plotted adventures, I simply create a setting and a handful of “adventure locations” and let the players figure out where they want to go and what they want to do there.)