You're looking for the types of plants that we often use with rain gardens (periods of dry soil followed by periods of very wet soil). What works can really vary by region, so I'd suggest contacting your local county extension office (just google "[your county] extension office" and it should pop right up) for a list of rain garden/wet soil plants that work well in your area. As an added bonus, the office is a mix of volunteers and professional, degreed horticulturists, and it's a free service that is WAY underutilized. They're awesome people who are there to help, check them out!

"They [low priced nursery/garden center landscape designers] are used primarily as a marketing tool to sell plants, often very common or less desirable plants, and what you receive design-wise may be very substandard." My sentiments exactly. One would likely get the typical "builder's plan" ... the kind of problem prone landscape that everyone can't wait to redo as soon as possible. The forum is good for giving ideas about general direction to take, or spot problem solving ... but it does not produce PLANS, which are essential for a quality landscape product. One has the option of drawing their own plans if they are DIY oriented and feel competent about it. (They can get advice from the forum on the mechanics of doing it. ) If not, as in every other project, they must hire a professional. But landscaping without plan at all will probably result in something less desirable than a 'builder's plan.' The approach one takes for a back yard depends largely on one's goals and objectives. Some people want total privacy, as in a private paradise ... they don't want to see any part of neighbor's houses. Other people want a more open solution and prefer not to block much view between neighbors' yards. Still others want to block sight lines between theirs and neighbors' outdoor living spaces while keeping a feel of openness at other places. One needs to decide on their goals in order for a plan to shape up that would meet them. In nearly every discussion of creating a privacy border of some type comes the advice to avoid a uniform hedge. I would take this with a grain of salt because often, especially when space is at a premium, it may be the uniform hedge that best fills the bill. Many people here are plant loving "gardeners" and can't conceive of a planting that is not crammed with as much variety as possible. A potential downside to that is more labor in upkeep, more space consumption, and a busier look. If that fills the bill, no problem. But there are advantages to uniformity in potential space conservation, a cleaner look and simpler maintenance. Landscaping is not 'one size fit all.' Develop your goals and set about to see how they can be realized.

Unclear what you are asking about. Pea gravel is not what to use for walking paths.

"The ground cover will be river rock ..." Sowe can all be using the same terminology, I'm thinking that what you're talking about here is the area beneath the swing ...? I would say the "gravel mulch under the swing will be river rock." In landscaping, groundcover per se, would be solid plants you use to cover the ground. With planting, I'm wondering if you're asking only about plants for the raised planter ... or if there are other areas of the yard that you want to use plants in. I don't see any landscape beds designated on the plan. If it's just for the planter, you're going to be limited to annuals (for color) and a perennial or two, and/or groundcover. Where bed edges tie to the fence, it will look a lot better if they tie at 90*.