Thank you! I longer hesitated while searching for a strategy for this one.
I first thought of painting, then of woodworking (Granite Falls statues that bring back much money!), but I finally decided to go on with flower arranging.
I read about pink orchids arrangement that become really valuable, so I decided to stick on that as soon as my sims were enough skilled to make pink orchids arrangements. They cost more than 800$ to make (I decided not to plant my own flowers because we only had 4 days) but bring more than 20,000 $ each bouquet on sale, via Plopsy or via the vendor table (with 300% marking).
I made 8 sims.
7 of them were always busy doing flower arrangements during the 4 days (each sim had her/his own flower arranging table) while being very inspired as often as possible to get the best quality. They were all "creative","art lover" and with a creative aspiration, so that they could have more inspired buffs, and "insider" to have the chance to get an extra buff because they were all members of a club (in order to get the "rally the troops" ans the "inspired club vibes").
The 8th sim was assigned to selling the bouquets with a vendor table, by doing yard sales all days and nights long, with 300% marking up. He was "insider","outgoing", "self-assured" and with a poularity aspiration, in order to max quicker the charisma skill and become a best vendor.
He also was the leader of the club I made at the beginning. (I was disappointed to see that "flower arranging" is not a possible activity for the club! I decided then to only set the activity of "being friendly" with everyone, which was perfect for my vendor and gave enough points to get quickly the "rally the troops" perk).
The sims sold a few bouquets via Plopsy but not very much, maybe 3 or 4 each sim.It is quite long and was way quicker to use the vendor table.
At the beginning, they practiced by making the cheapest arrangements in order not to lose too much money. It would have been impossible to sell all the bouquets made in 4 days by making them h24 with 7 sims, so I decided not to sell the cheapest ones. As soon as they were able to make pink orchids arrangements, I kept selling those and let the other ones.
(I finally lost a bit money because some of the bouquets could not have been sold,including pink orchids, but that was really not much compare to the big amount of money my sims finally got!)