Tel Aviv is fantastic, friendly and great nightlife.
Jerusalem is a must. Wailing wall and Dome of the Rock are the most common places to see, stay on the tourist trails. Don't bother with Bethlehem or anywhere in the zone, but if you must, only do a guided tour.
Masada was superb, en gedi and the dead sea was OK.
I go monthly, if you like military history then a trip up to the Golan Heights is worth it, especially to see the Valley Of Tears where the remnants of a huge Syrian defeat still sit. There is a decent winery up there and it's on the way.
The nightlife is great and to caveat what ViVictaVis said..women under 30 are generally very fit, the sun dries them out after that.
Don't forget to bob about in the dead sea for a while.
Nazareth is a a bit of a sh1t-hole but worth a visit if you fancy a bit of Bible-tourism and would like to see a safe Arab town. Haifa is good for a night out and has a bit more of a relaxed feel to it than a lot of places in Israel. Jerusalem is simply unmissable.