Garden lighting. Layered rather than flooded path lighting for safety, uplighting into trees, and low-level light around seating. Done properly, it makes a garden usable after dark. Done badly, it makes it look like a car park.
Power and water. External sockets, outdoor taps, and cabling run at design stage rather than surface-mounted afterwards because nobody thought about it.
Garden rooms and outbuildings. Bases, services, and groundworks, and the landscaping that ties them into the rest of the garden.
Every one of these things is far cheaper and far better if it is designed in from the start.
Lighting cable and power runs are trivial to lay in an open trench during the groundworks. Retrofitting them afterwards means lifting paving you have just paid for. The same goes for a gas run to an outdoor kitchen, a water supply to a tap, and a level, load-bearing base for a hot tub which weighs a great deal more than people assume once it is full and occupied.
We ask about all of it at design stage, even where you have no intention of building it this year. Running the cable and capping it costs almost nothing now and saves a great deal later.