For example, if there is a large tree in the middle of some grass, make it your focal point by building a seat around the base and working from there. Gravel is a great material for a low-budget garden. The cheapest way to buy it is in bulk from builders' merchants. They deliver it in tonne bags as well as smaller individual bags. Don't try and tackle the whole area - maybe focus on one area and lead the eye towards this with clever planting.
Ordinary trellis can be an attractive feature when painted with a timber wood stain.
It is L-shaped around the house, the two long arms being over 100 feet long each. The soil is heavy clay, and the plot is almost totally flat. We would like a rill. But generally you would not be dealing with sites that are 100 foot long at each stretch. The secret, as you say, is to create a series of gardens.