Formal:
* Fashionable: This is a classic synonym for trendy.
* Stylish: Implies a high level of fashion sense.
* In vogue: Indicates something is currently popular.
* Modern: Often refers to contemporary style or design.
Informal:
* Hip: A slang term for trendy and cool.
* Hot: Used to describe something that is very popular and desirable.
* All the rage: Indicates something is wildly popular.
* The latest craze: Describes a trend that is very recent.
* The next big thing: Implies something is on the verge of becoming trendy.
The best word to use depends on the context and the tone you want to convey.