Not to beat a dead horse on the menu topic, but the concept of a menu as any mechanism allowing users to select from a fixed set of actions when prompted, without having to type long commands, occurs to any programmer who writes code for even pretty simple user interactions. In fact, it's a lot easier to create such a UI than to write code to parse command lines. I doubt it would be possible to identify an earliest use (or cluster of uses) of such UIs among all the programs that were created in the early days for TTYs or dumb terminals.
On the topic of the menu *bar*, though--as Tom points out--Xerox PARC seems to have settled that frontier first, though they didn't deploy it commercially (Xerox Star) until 3-4 years after the UCSD p-System, with its drop-down menus, was out on the market. In one of my interviews of him, Ken Bowles, the leader of the UCSD project, didn't recall any influence of PARC's ideas on his team, but you never can be sure about how memes might worm their way into various places.