ArchiCAD library does offer some 2d trees and people that you could just add to the 3d Document. Send backwards, forwards etc on elements works fine as longs as the Uncut Elements have a solid background (or -1 window background).
I use a few choice 2D objects in my sections elevations for a little added detail that I can't really be bothered to model 3D and I can hide them partially behind 3D elements, same as if they would be 2D fills.
The benefit of 3D document is also live connection to the model, small changes will reflect right away and if you manage to limit the 2d patch work to the bare minimum, you will have little time to spend on updating the impression when your design changes.
With renders, which will also do this fine, you do need to go in Photoshop to overlay colour and sketch outlines at the very least.
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5