18mm is decently wide already (28mm in 35mm terms).

When I was shooting film 24mm is my favorite and mostly used wide angle lens. I found 20mm a little too wide for me to manage. But there may come times when you wish you have wider angle lenses (like when you go visit a museum or you are taking a shot on a cliff's edge, etc). In case you might be interested:

The Nikon 12-24 is great, but over priced in my mind. The Tokina 12-24 is also an optically great lens. It's better built too.

The Nikon 60mm is a great micro lens for shooting flowers and insects. It's SHARRRRRRRP. The Tamron sp 90mm is also great. It's light weight also. But with the 1.5x ratio you may find the 90mm's angle of view too narrow. The new Nikon 105mm VR is super. I haven't used that one, but it has good reviews and theoretically you don't need a tripod for micro shot with the VR on.