There are a few ways to get money reasonable quick, but they require a bit of investment of your time and money. The first method requires much more of your time than the second one, but you also get much higher profits. These methods work all-year round (I think)
1.) Make golden/silver cloth out of flax (or cotton) - Requires Sewing Studio
Initial investment:
Sewing Studio: 40,000 G
Flax Seeds: 260 G
ROI: ~ 14 golden linen cloth units sold | ~12 golden cotton cloth units sold
Since you already have the Sewing Studio, all you need to do is plant a ton of flax. After harvest, go into the Sewing Studio and make Linen Thread out of it, using the Spinning Wheel. 1 unit takes 15 in-game minutes to make, so you can just queue up all you got (30 units is the maximum per queue).
When the thread is finished, you need to dye it. Dying takes the longest (both time and effort wise): 6 hours and 50 minutes (in-game) per unit. If you want it to be finished in some reasonable time, I suggest you make queues of 5.
After the dying process is finished, you just need to make cloth out of it. Making cloth only takes one hour.
Depending on the quality of your cloth, you can sell it for (approximately) these prices:
Linen:
- 1 Star: 3000 G
- 3 Stars: 5000 G
- 5 Stars: 7000 G
Cotton:
- 1 Star: 3600 G
- 3 Stars: 6000 G
- 5 Stars: 8400 G
2.) Make Chilli Powder out of Chilli Pepper - Requires Seasoning Shed
Initial investment:
Seasoning Shed: 40,000 G
Chili Pepper Seeds: 600 G per unit
ROI: ~ 84 Chilli Powder units sold
Again, you need to plant lots of chilli pepper. After harvest, go into the Seasoning Shed and make chilli powder out if your pepper, using the seasoning pot. 1 unit only takes 15, so it's a rather fast process.
You can sell the chilli powder for these (approximate) prices:
- 1 Star: 480 G
- 3 Stars: 800 G
- 5 Stars: 1120 G
Before you sell your stuff, check if a trader wants cloth or spices (you can sell them for a bit more). Also be aware that if you sell too much of a certain category, selling prices will lower for that category at this trader.
Either wait for the next day or sell to another trader.
If you already have your own trading spot, you can just fill it up with cloth/spices and sell everything. Prices won't go lower and you get a notify when you sold everything.