"Happy" is how you say "开心" in English.
If you want to express happiness in a more elaborate way, you could use phrases like "overjoyed", "elated", or "ecstatic". These words connote a stronger sense of joy and excitement than the simple word "happy". However, if you are looking for something more casual or informal, you might just use "happy" by itself. It's a very common word and can be used in many different contexts, so it's versatile and easy to understand.
