Do you think my 03 Avalon can handle the drive from San Diego to Big Bear Lake and back?

Easily.

As long as there’s nothing wrong with it, why not?

When was the last time you replaced the water pump or timing belt? I’ve had the water pump fail on me with this exact model, and it’s a pretty big job. But otherwise, these cars are awesome.

Come on now, she’s just getting warmed up.

Probably, but how long is a string?