How can I improve the garden to this house?

When I'm ever in this situation, I just put kids stuff or planter boxes. Mainly I just scroll through the outdoor activities and see anything that seem like it fits. Also sometimes, I'll go for a pool to take up space without much effort or greenhouse if I have a green-thumbed sim (which i most often do). If your going for a minimalist/modern vibe, don't put anything and get a different tone grass terrain paint and make it look like a freshly mowed lawn.