(The problem I have selected is not necessarily the most important one in my life; I confess I tried to pick one that would let me produce lots of answers, be reasonably relatable for others, and not leak more personal information than I am comfortable with.)

Problem: I don't have so much money that I feel able to retire with confidence that I won't run out. (I am still rather younger than the age at which most people retire, but I would like to be able to do it early, although I might choose to retire only partially in various ways.)

Solutions (I am including partial solutions that definitely don't solve the problem completely, and unlikely solutions that probably don't help at all but could, and stupid solutions that might solve the problem or at least help with it, but at other unacceptable costs):

Adjust my level of paranoia about future adversity. (Before I am comfortable retiring, I need to be confident that even in quite improbably bad possible futures I will not run out.) [4]
1. Take up Buddhist-style meditation or Stoicism or something of the sort, cultivating an inner conviction that even the worst that can happen (whatever it is) is survivable.
2. Persuade my wife to take up with someone else and bring our child with her, so that I no longer have to worry about the wellbeing of other people besides myself. (This has obvious adverse consequences of its own. On the other hand, it is also an answer in category 2 below as well as in this category 1.)
3. Do extensive modelling of plausible financial futures; the how-much-do-I-need heuristics I use are fairly conservative, and perhaps looking at it more closely will show that I don't need to be so careful.
4. Change my current policy of assuming (almost certainly very counterfactually) that my parents will be immortal, squander all their money, choose to leave it to a dogs' home rather than to me, etc. (I currently make that assumption because (a) I think it's healthier psychologically not to see one's parents' deaths as an opportunity, (b) I think it's healthier psychologically to take responsibility for one's own financial security, and (c) I don't like relying on things that are outside my control. I don't actually think changing this would be good overall.)
Reduce how much money I feel I need in retirement. (Most of these, if implemented immediately, also increase how much I can save, which also helps.) [11]
1. Get out of the habit of eating fancy food. Buy the cheapest of everything and get used to it, find a repertoire of frugal but edible meals, stop cooking unnecessary things like cakes, etc.
2. Stop buying books.
3. Stop upgrading computer hardware; progress has slowed a lot, after all.
4. Move to a smaller house, thus reducing utility bills and certain local taxes.
5. Don't have a house at all; live in a caravan or car, or a tent, or become a squatter. (This may also be a way of making 1.2 happen.)
6. Move to an area where things are cheaper.
7. Move to a completely different country where things are much cheaper.
8. Completely give up driving; walk, cycle and use public transport.
9. Tell my child not to expect any more than the barest minimum assistance from me -- no paying of university fees, no help buying a house, etc. (This may also be a way of making 1.2 happen.)
10. Abandon hobbies that cost money, replacing them with ones that are free or nearly so.
11. Contract a disease that greatly shortens my likely lifespan but doesn't require expensive care before my tragically premature death.
Acquire more money. [19]
1. Just keep on working and saving for a while.
2. Negotiate aggressively with my employer for a higher salary.
3. Shop around actively for jobs that might pay more.
4. Identify changes in my skills, behaviour, etc., that would make me more likely to be promoted, and implement them.
5. Switch from tech R&D (which is pretty well paid and mostly fun) to finance (which is better paid but I would probably find much less fun).
6. Move to a city where salaries are higher. (In the UK, currently working in Cambridge, that probably means London. Note that cost of living there is really high so it would not obviously be a win overall.)
7. Move to a completely different country where salaries are much higher. (That would mean the US. Not the same completely different country as in 2.7 above.)
8. Wait for my parents to die.
9. Turn to crime. If sufficiently unprincipled, I could probably find several ways of getting hold of quite a lot of money.
10. Return to my long-ago-abandoned career in pure mathematics and somehow solve one of the "Milennium Problems". (Not a strategy with a very high probability of success.)
11. Gamble (e.g., lottery tickets). Negative expectation, of course, but it could solve this problem. Probably more likely to than trying to prove the Riemann Hypothesis.
12. Start a company and lead it to a successful exit.
13. Sell some organs.
14. Sell a lot of my existing possessions. (Most things sell second-hand for much less than they were bought for new; this isn't a great way to raise money.)
15. Get divorced and marry someone very rich.
16. Become an artist and persuade the art world that my creations are (or, almost as good, soon will be) super-valuable.
17. Persuade my wife to take a higher-paid job.
18. Persuade my child to take a job and hand the income over to me. (Difficult to make much money this way at her current age, even aside from ethical issues and the like.)
19. Stop donating to charities.
Get other sources of money besides salary and investment earnings/growth. [8]
Change the world in such a way as to invalidate (pessimistic) assumptions implicit above. [5]
1. Persuade the government of the country where I live to implement a robust universal basic income scheme.
2. Develop a superintelligent AI, sufficiently well aligned with humanity's interests that the near-magical technologies it develops usher in a new post-scarcity age.
3. Develop a superintelligent AI, which doesn't give a damn about humanity's interests but is sufficiently well aligned with mine that the near-magical technologies it develops, or the near-magical criminal schemes it thinks up, or whatever, give me everything I need for the rest of my life.
4. Lead a communist revolution, after which private property is an irrelevance and the glorious all-benevolent state will ensure that everyone has everything they need.
5. Create a new cryptocurrency and persuade everyone to start using it instead of dollars, RMB, bitcoin, etc.
Other. [3]
1. Die. My financial needs for retirement are then zero. (This is another one that has a few downsides.)
2. Abandon my preference for not needing to work. I still won't feel comfortably able to retire, but I won't care any more.
3. Once such a thing is possible, get uploaded. Virtual-me is probably cheaper to run than real-me.