By default there’s not much, fluxbox allows you to change its style, the wallpaper and there’s the slit, but for anything beyond that you have to resort to other means.
You can replace the default fluxbox taskbar or use an additional taskbar application. To replace it you have to hide fluxbox’s taskbar, either from its menu (makes changes to the init-file) or manually by editing the init-file and restarting fluxbox.
If you want the Systray to be taken over by your external taskbar, you need to disable it in fluxbox’s taskbar by deleting Systray from sys[0]taskbar in your init file.
Some of the alternatives shipped with debian:
The most prominent tool is probably conky, with it’s LUA API its capable of drawing impressive things on your desktop, all of which is not interactive though. For a great variety of clickables and draggables you can install gdesklets.