You don't have to travel in order to complete the gardening (or fishing or cooking) challenges. It does make it a lot easier though. Basically, the challenge to plant "all" plants is now to plant 19 different plants. Without traveling, that means getting the opportunities for eggs and cheese, then burger and steak, then omni plant in order to get 19. With travel, you can just pick up some foreign plants and not have to worry about the skill opportunities.
The fishing and cooking challenges for "all" are the same, with fishing instead of having to catch the hardest fish in the home town, you can just catch some of the easy fish in a foreign location. The Cooking challenge is made easier just by having the expansion, since you can learn the foreign recipes just by watching the cooking channel on TV or swapping recipes with a natural cook without ever having to travel, or if you have multiple sims one sim can travel, buy multiple copies of the foreign recipes and bring them back for the other sims to learn.