@Gwiniel I have the Outdoor Retreat GP which adds several plants (probably the most out of any of them besides Seasons, but I don't know for sure) as well as the Herbalism skill, which lets you create concoctions from plants.
Personally, I garden mainly to harvest & make money, because with the amount of plants I use, it would take ages for Sims to walk around the lot gardening them all if they were decorative, and I just don't have the patience. I might try it when some of my Sims are elders and have some time to spare. :D
If you really love making decorative things, you might look into the Romantic Garden SP. I don't have that one yet, but someone on here might be able to give you more information about it. As far as I know, there aren't any ponds in Sims 4 (hopefully one day!), but you can cleverly use the custom pool tool combined with rocks and plants to get a similar effect. I'm sure there are some YouTube videos about it. :)