If a coffee shop is so busy that you won't have a place when you get back then take a break and come back when things are quieter. Or pay for a coworking space.
(I remember trying to get a table with friends at Blue Bottle years ago... each one was occupied by a loner staring at a laptop, nursing a coffee, and ignoring everything else. It was sad.)
Most studies have found that most of the time that people believe a drug was added to their drink, the only drug they actually consumed was alcohol. Alcohol is by a long run the most common date rape drug; it is far more likely that your attacker will trick you into drinking more than you realize.
So if somebody is buying you unfamiliar cocktails, merely guarding your drink against adulterants can leave you vulnerable to the cocktails containing far more alcohol than you are aware.
http://en.wikipedia.org/wiki/Date_rape_drug#Alcohol http://en.wikipedia.org/wiki/Date_rape_drug#Media_coverage_a...
It's also not unheard of for same-sex drug- or alcohol-based assaults to occur.