It's been many years but the whole beach area of Tel-Aviv basically has a similar feel to South Beach, Miami. Go visit Jaffa, as well as Binyamin outdoor mall. The falafel at some restaurant named Gina was also out of this world. To be honest, you won't have a problem finding a good time if you stick to the downtown area. There are also crazy raves every weekend if that's your cup of tea.
IMHO, you should make an effort to go visit Masada and Jerusalem, both of which have ancient architecture and are definitely worth the visit, whether you're religious or not. If you go to Jerusalem and the police let you, try and get up close to the Al-Aqsa mosque, if only to watch the burqa harpies shriek at you for defiling their "holy place".
Even with 1.5 days you can visit pretty much any part of the country you want since it's so small and you can drive anywhere. The Dead Sea is also worth a visit/swim, as long as you don't have any open cuts.
A tour of the various farms in the middle of a desert is definitely a modern marvel as well and worth a tour. The technology they use to grow shit in the middle of the wasteland is definitely miraculous.
Just keep in mind that Israelis, particularly dark skinned ones and especially the Arabs, have a very poor concept of social etiquette, at least by North American standards. Very gruff and outspoken, which can sometimes be mistaken for aggressive. Also they have no clue about how to form a line, or get on/off an elevator.
Also, not sure if it's too late, but if you haven't booked your flight yet and have a choice, fly on a Friday. Ultra Orthodox won't fly on Fridays so you won't have to deal with their stupid religious bullshit on the plane.
EDIT: And seriously, do not say or do anything stupid or sarcastic at Ben Gurion security. They seriously don't fuck around and make the TSA look like a bunch of Down's kids.
