* Enter: This is a very straightforward and neutral synonym. "He entered the car."
* Go into: Similar to "enter," but slightly more informal. "He went into the car."
* Climb into: This implies a bit more effort, perhaps due to the car being high or the person being small. "She climbed into the back seat."
* Hop into: This suggests a quick and casual entry. "He hopped into the driver's seat."
* Slide into: This implies a smooth and easy entry. "She slid into the passenger seat."
* Settle into: This suggests a comfortable and relaxed entry and settling in. "He settled into the driver's seat."
The best choice will depend on the context of your sentence.