Is this a single residence, or multi-family housing? The areas with chairs look a little bleak, but that may be the angle of the photos. I'm guessing that if you sit in the chairs you are gazing at the woods? If that is the case the areas probably feel reasonably comfortable and serene as they are, but if the view is of a road or parking lot, it wouldn't hurt to have some groupings of large planters filled with flowering plants or shapely foliage plants.

It looks to me like the emphasis is on low maintenance -- very low maintenance -- and anything you add may disrupt that concept. So before making changes, consider who will care for things after you are gone. Pots full of plants that die after you have left will not add to the scenery, for example.

Trails -- to make a trail "warmer and more inviting" it helps to have a destination. Something very simple, like a crude but solid bench placed in a shady grove or on the edge of a meadow or other viewpoint can be sufficient reason to encourage a walk.

Trails should be just trails; not gardens, IMO, and are maintained by trimming encroaching branches and making sure the path is reasonably smooth -- no ankle-twisting potholes or overly steep slopes. Naturally occuring stones and tree roots are part of the experience, so leave them be.

Which photo shows the transition from yard to woods?

This reminds me of a project I did for a bed and breakfast property. From your pictures it appears that the paths through the woods are well used, compacted and dry. Where possible, the path should be two persons wide and allow a riding mower to traverse it in the fall to blow leaves aside. Easy quick solution to heavy leaf drop.

I do not understand your thought "about distingushing lines between the yard and woods". Suggest you forget the idea and not get 'cute' here. It is fine as is.

As Catkim says above, you need destination points along the path. This is done with a few rustic benches at strategic spots.

Suggest you send the owners on a hunt for rustic/interesting birdhouses to hang on trees. This is an ongoing project which my former clients have enjoyed both in the searching for and enjoyment of nesting birds.

As to the rest of your questions...are deer and rabbits a problem? Probably. This limits your planting choices. Locate a list of deer resistant plants and work from that. Rabbits have an acquired taste for almost everything and dine comfortably on what strikes their fancy at the moment.
It takes some thought and study to successfully plant in these types of situations.

Hi, thanks, the suggestions are much appreciated. The benches are a great idea. Yeah its kind of a bleak view, might have something to do with my buddy moping around. Kidding.

I don't really have any good shots of the transition, the yard pretty much turns into the woods haha. There is an occasional deer but haven't seen many rabbits. Guess I'm going to have to pound this out for a while and see what swells about. I'll take the suggestions on finding deer resistant plants. Any other comments are welcome.

"side 1" the double trunk tree & a urn with something in it-boring!! Maybe 3 low growing plants hostas something on plant list that critters won't eat, daylilies if get enough sun & some bright perennial in the urn. "Side 2 of house" nice multi- trunk tree & AC ,bleak, needs something that grows about 2 ft tall to make AC less noticeable & something low & easy to care for under tree. Is that all his house? If it is he needs an arbor or trellis to break up all that space with a seating area. Chairs & table are just plopped there- think "outdoor room" need "wall"= 3-4 plants to make it feel more homey, something to make it inviting- even a piece of driftwood on the table with a glass bird from Thrift shop would help. Right now it looks more like a camp ground. If you want to do something about trails I notice there are branches down, could try using them as a border with a plant or 2 just as you enter woods. Does the guy have hobbies, into shells from ocean, hunting, etc. Find couple of Garden Art pieces that say he lives there. Frog, turtle, iron sculpture, cement rabbit etc. Look on Garden Junk site for some ideas. A rusty circular saw blade hung on piece of barnwood for guy that makes things from wood for hobby, Interesting rocks among the plants by the 2 trees mentioned above, a rusty old rake hung on a wall of house with dried flowers & rope with knots he makes hanging down. Got to give the place some sense that a warm human lives there., Is he into boating, oar hung on wall with fish netting, floats & couple of old fancy fish hooks either plastic or wooden. Etc. Good Luck. If the guy doesn't want to spend money, can you gather rocks from somewhere & give him some borders with just couple of plants each place at least looks like he is trying!!

sunnyca- Thanks alot, those suggestions are inventive and inspiring. The prof is a mellow older guy that's a big time viola player and antique collector, so I'm a bit cautious on what to add but some of these ideas may lead to something that might work. Much appreciated.