Apparently, "sene'yla" is coffee mixed with ginger ale. I typed "Vulcan drinks" into Google to find out what a Trekkie like Nimoy would order and found that. I quote from the page of vulcan recepies:
Suggested uses: Read a Vulcan related book. I've found that after drinking glasses of sene'yla while reading Vulcan related books, I will associate this drink as a "Vulcan" one. For that matter, drink this in a Vulcan-like enviornment to enhance the meaning it holds.
The Internet is truly a wealth of exotic and meaningful information. Each new nugget of knowledge it brings to me makes me a little bit stupider.