With the big root, you may want to tear out the bush and try and kill the root just to be safe, because it could grow enough to damage your foundations and sidewalks at some point. You could check with a local nursery and see what they say just to be sure, but if it were me, I'd just get rid of it.
As for the tranquility pond, I'd go for it! I think water features are just the greatest things and they add such drama to an area.
Plant wise, I'm not good at growing stuff either, but I do know some things that are easy growers. Morning glories grow fast and easy, but I think you need to plant them in the spring. Bulb type flowers (tulips, irises) are pretty easy, and you can plant them in the spring or fall. A few years ago I found some unusual bulbs that I planted and then had to move before they came up, but my point with this is that there are a lot of bulb type flowers to choose from.
Another thing is you can drive around your area and see what is growing in other peoples yards or just growing wild around your town. If you know someone that has property that you can dig up wild plants from (or if you are very sneaky and you can dig them out of empty lots or on the side of a back road) dig up some native plants that you like and plant them in your beds. They won't take a lot of extra care, because if they are growing wild they'll grow without much help.
If you want some added color you can stain your sidewalk. I don't know how much concrete stain costs or how to do it, but I've seen the end result in a few areas and it really makes a dramatic difference.
Then there are trellises if you want some height near the house. You could paint them any color you like and plant some vine type plants or string them with Christmas lights for some instant evening drama.
These are just off the top of my head, so I'll keep thinking.