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In a discussion on Google Plus, it seems that most people run carousing so that you get initial XP for recovering treasure and then more XP from spending it on carousing (which is obvious, now that I think about it). In other words, carousing is an XP multiplier, at the cost of potential complications. Thus the structure of carousing is actually more like 1 GP = 2 XP. So from now on that is what I will do.
In addition to carousing, there are several other activities for spending GP and reaping XP that are structurally similar (get a bigger payoff, but also risk complications if a saving throw is failed). The options are:
Each of these activities has some potential minor payoff if the saving throw is passed. Successfully rendering judgment might improve the reputation of a cleric and make it more likely that NPCs will seek assistance from the cleric in the future. There might be a positive reaction bonus from the NPCs that participate in carousing. Magic-users might be able to identify some aspects of a mysterious enchanted item. Fencing hot property or finding someone to buy controlled substances from (like poison) might be the outcome of successful streetwise. None of these side effects are guaranteed though. The major benefit of carousing and friends is as an XP multiplier: 1 GP = 2 XP.
In the most recent version of carousing on Jeff's blog, the saving throw required to carouse without incident is poison. That makes sense, as one is trying to resist the effects of partying hard. A save versus spells might work for magic-user experimentation, but there is no appropriate save for either judgment or streetwise. These are both social activities, so a charisma check might be logical, but structurally the check should be about experience (and thus level), not inherent talent. Given that poison is generally the most favorable saving throw, the solution I have come up with is to have any of these multiplier activities require an abstract saving throw. The player may just use the most favorable save available (this preserves the numbers for carousing while extending the same chances to the other three multiplier activities).
Thus, in all four cases, the procedure is as follows. Roll 1d6 and multiply by 100 GP for the cost. Then, take twice that in XP. Make a saving throw (use most favorable). Upon failure, roll on the appropriate complications chart. No more than one such activity may be attempted per week of downtime, though other activities (such as working on a scroll or training retainers) may be pursued during the same week.
Optional rule: characters may attempt an off-class activity, but it does not function as a multiplier (that is, the activity results in 1 XP for 1 GP). I'm really not sure about this, to be honest, but I kind of like the idea that a fighter can steal a peek at a grimoire and attempt to puzzle through some of the spells at great risk. It could even have some interesting game consequences (we have to identify this magic item, but there are no magic-users or sages available). I'm not sure that any player would ever take up this option, though. Also, I think other classes should be able to carouse, but I don't like the idea of always sharing the fighter's toys. So freedom for anyone to do anything but with lesser payoff seems like a decent option.
I need to write up complications tables for judgment and streetwise. Success tables for all four might be fun too.