What a lot of you are talking about with continuously improving the plant quality and waiting for the opportunities to bring in excellent and outstanding produce are not considered "challenges." They are opportunities that come to your Sim via the telephone. If you're thinking in terms of the
Immortal Dynasty Challenge, these opportunities are part of the 6 unique opportunities that are required to eat Ambrosia.
The skill challenges are found in the gardening skill journal and are as follows:
1. Master Planter - Must have planted 19 types of plants that can be planted in the game.
2. Botanical Boss - Must have harvested at least 75 perfect fruits and vegetables.
3. Master Farmer - Must have harvested at least 650 fruits and vegetables.
These skill journal challenges are what must be completed in addition to reaching level 10 in the skill as a requirement for eating Ambrosia.
Increasing the quality of plants is important, of course, but it's also important to just do a lot of harvesting and hunting for seeds. You can purchase fruits and vegetables from the grocery store to plant, but all of the Special Seeds have to be found. If you cannot travel for whatever reason and are limited to your home town produce, you'll have to complete the string of challenges for the Omni plant, which will allow you to plant eggs, cheese, steaks, burger patties, and the Omni seeds.
For the Immortal Dynasty Challenge, all of these things are important and have to be done. The best advice is to start early, have a good sized garden, and tend it every day. Fertilizer is important as it shortens the growing time and increases the number of fruits produced.
Carl's Guide on fertilizer may be helpful here.