“Casual but nice” usually means the host wants you comfortable, but not accidental. The safest answer is one polished element paired with something relaxed.
Formula one
A midi dress with a flat, sandal or low heel. Choose a dress with enough shape that it does not need much styling.
Formula two
Fluid trousers with a fitted tank or knit and a light blazer. Remove the blazer if the room feels more casual.
Formula three
A midi skirt with a simple tee or sleeveless top. The skirt supplies the polish; the top keeps it easy.
The rule
If every piece is casual, add structure. If every piece is dressy, relax one of them. That tension is usually exactly what the invitation means.