- Rent a car when you land (airport rentals are open on Shabbat) and head directly to the North.
- In the north, you could base yourself in Haifa, or a number of other places that may be more convenient. Once you figure out what you want to do in the North for 2-3 days, you can choose where to base yourself.
- Don't spend any extra time in TLV... or maybe take 1 day away from the North and come to TLV a day early. I'd imagine you'd have enough downtime (e.g., evenings) to get a feel for TLV while you're already there for the conference. Beaches will probably be too cold in December, and if you're not into nightlife, no point in spending a whole lot of time there. There isn't so much in TLV in the way of "sites."
- After the conference, head to Jerusalem. Jerusalem has tons of stuff to see and is fascinating, and a day trip just isn't going to even scratch the surface.
- Better yet, can you just stay in Jerusalem after the day tour, rather than going back to TLV and then back to Jerusalem?
- Consider a day trip to Masada and the Dead Sea from Jerusalem while you're there during the last 3 days.
- Ditch the car when you come back to TLV. You won't want to deal with it in TLV.