"A trip down the lake" as it stands on its own... could mean a trip to the lake, or lakeside ...or alternatively it could equally mean a trip literally down the lake (in a boat).

For me would usually mean a trip in a boat

Thank you Chris.

The writer isn't making it totally clear but the assumption would be a trip down the lake by boat because that would be more normal. I think if a writer wants to give the idea of a journey by car or foot he would have to add these details or change the verb to, for example a walk or a drive