The 17-55 is a very nice lens, but it isn't really going to expand your capabilities much, except for wider aperture.
If you like shooting flowers, you may enjoy a 90 or 105mm macro lens, which would also double as a fast portrait lens. The Nikon 105 VR is great, but so are the Sigma and Tamron macros in that focal length. If you're inclined to buy used, you should find a good selection here and from on-line retailers.
My next choice would be to go for a longer zoom, such as a 70-300, if you frequently find that the 18-135 doesn't get you close enough. The Nikon 70-300 VR or the Tamron 70-300 SP VC are good choices.
A super wide zoom, like the Sigma 8-16 would be fun, but it isn't going to do much for you for flowers or grandchildren.
Depending on your budget, you may be able to get more than one of these, especially if you buy used.