When I design a garden, especially a small one, pretty much the first thing I consider is where the seating area is to go. Seating areas have a huge bearing on the overall garden layout and they establish how key areas are to be accessed and how the garden will flow together.
Some of my clients like to sit in the sun and some in the shade and views always vary as to how they actually want to get around a particular seating arrangement. Is it a large table and chairs for ten people dining 'al freso'? Or is it a simple two seater bench to be placed amongst woodland planting under the shade of a tree? When thinking seating design, try to see it as part of the bigger picture, rather than it being a plonked afterthought. A successful seating area will make both a destination point and a strong visual feature in the garden. It therefore needs to be integrated into the garden style to make the design cohesive overall. Be as generous as you can with seating as there's nothing worse than feeling squeezed into an area. If chairs are moveable rather than fixed make sure there's plenty of room to pull them back to comfortably get into the space too.
There are literally hundreds of different types and styles available to buy off the peg, some entirely wood or some a combination of wood and metal such as lightweight aluminium. Other materials include woven rattan/wicker, synthetic woven materials, plastic and acrylics. I find cushions are essential if the seating is to be comfortable in the long run, but these can easily be carried out from the house on a dry day and used to help style the garden too.
When building retaining walls in gardens I always see it as an opportunity to incorporate some built in seating which can look great, and give the garden a sense of permanence and be an efficient use of space too.