Have grab bars installed, to reduce the risk of falls and assist your senior with mobility issues or balance problems. Typical locations are near the toilet, tub and shower. Note that in senior housing, railings are installed all along halls; the idea is to put them wherever they may be needed. Especially on any stairs!
Provide adequate doorway and hall widths for a wheelchair. Seniors may not need a wheelchair, but may use walkers to get around. These may require some additional width to doors to get through without banging knuckles or the walker itself against door trim. No-threshold showers are needed for wheelchairs, and they can be designed to look like a spa.
Consider that their eyesight that may not be the best. Task lighting in potentially shadowy areas and adjustable light levels will help tremendously.
Many seniors have arthritis or limited mobility and may need larger pulls or levers that are easier to operate. You might have them test the faucet handles and drawer pulls to see if they will work for them.