But, I could see great benefit in a framework for those other things. Something along the lines of, "You get 1 effort point for each weak you spend not adventuring." Then you could tie various mechanics into those effort points. Crafting an item, running a business, building a kingdom, etc.
This makes me think that when a DM proposes a framework for downtime, is it often
training rules, which are a series of "negative" rules, i.e. house rules that take away something that standard rules normally give you automatically, and then say "if you take downtime, and declare you use downtime doing
this, then you can have
that back".
E.g. the framework may say something along the line: "you need to spend one week of training per level, in order to level up and get all the benefits", or alternatively be more details like "one week of training required for each skill rank increas, one month required for each feat, two weeks for each new arcane spells etc."
I am not against this approach, but clearly it requires everybody to accept it in the first place, and it does not allow players to make their choice individually. I don't think it ultimately adds much to the game, you just end up making your next level choices, calculate how much time you need to be off and declare it, get pissed off if the DM decides you're interrupted. Some character classes might be penalized but this will probably just set the minimum downtime for the whole group (assuming they level up together, which is often not the case).
The opposite approach of having "positive" downtime rules is much better, but doesn't apply to levelling up, it apply to something
in addition.
And here are two routes to follow (although both can coexist). You can allow downtime activity to grant additional benefits at the cost of only the time spent, or you can require some additional cost (typically money).
The latter is the approach used by the crafting rules in 3ed. This makes it a
conscious choice: you spend something to get something in addition. Ultimately, these are supposed to even out, so it's really possible for one player to take advantage of all downtime and for another to totally ignore the option. However, the downtime spent itself tends to become secondary to the other cost, and even annoying to some players, in fact later 3ed material spawned rules for splitting up or totally ignore (e.g. craft points) the time required (these are rules for those who
don't like dealing with downtime!).
The former carries the danger of unbalancing characters if one player uses downtime as much as possible while the other is not interested. Would you allow the first player to get better and better at some skills just because he says "I spend all my downtime climbing trees", while the other player doesn't?
However, I am actually very interested in this approach,
as long as it is allowed for benefits that are themselves mostly confined to downtime. That is, don't let a player build equipment, learn spells, or add any other character "feature" just by spending downtime (unless she has an additional cost), if those features be used in the exploration and combat phases. But allow it, if the features gained are usable in the interaction phase (or any other downtime) and are strictly non-mechanical.
Examples can include developing contacts with NPCs, working towards memberships in restricted groups, or even gathering some specific knowledge, as long as you don't just treat it like granting a +1 to a lore skill, but perhaps in the form of answering specific questions, e.g. "I want to learn more about the Kingdom of XYZ" then next session hand out a page of written information that the player can read on the subject. Also learning new "recipes" for alchemical items or herbalist potions can still fall within the scope, since anyway the PC will need to pay to craft them.
Investing money in a merchant business or building a castle can also fall in this framework fine, but only if the campaign is at a stage where money doesn't matter anymore (which would be impossible in 3ed), otherwise you have to make the investment balance out to a zero monetary benefit.
How does this all make sense? Well it does for
some players, on the ground that maybe if player X is interested in spending downtime to build a castle, the reason has to be
because he is interested in building a castle, not because he is interested in multiplying his gp stash or getting a +1 somewhere. That player is going to be rewarded just by the DM providing simple rules on how to design a castle, and then after a few session hand out a picture or a map for the finished work. Of course
other players aren't interested, but this way they aren't penalized if they ignore the downtime optinoal "rules". IOW, make the player rewards herself without a net benefit, make the castle, the contacts, the membership or the merchant business spawn a quest later ("your castle is under attack!", "your spy in Candlekeep was arrested and you're afraid he might name you") as well as occasional non-monetary, non-combat related benefits ("you can retreat to safety in your castle during the undead invasion", "your spy in Candlekeep will let you have a key for the library").