Obviously, you should pack conservatively for a long trip, and if you do forget something, it’s pretty easy to buy it in Israel (though prices may be higher). Airlines are now charging a lot for baggage in excess of the weight limits, so beware! That said, I confess that I’m a congenital excess packer—but even I got it down to 42 lbs. on a two-week spring trip. If I can do it, you can! Even in the summer, do not pack too many pairs of shorts—at most holy places neither women nor men can gain admittance in shorts. Overall Israelis wear shorts less often than Americans, particularly women. Shorts are fine outside of religious sites, but the shorts-and-sneakers look may identify you as American if you care about that. On my most recent trip, I showed up looking like that, and a dear Israeli friend asked—with a smile—“could you look any more American?!” On days when you will be touring religious sites, legs and shoulders must be covered, for both men and women.
For women, that means skirts or dresses below the knee or long slacks are required, especially at Muslim sites. A shawl or light shirt or jacket that can be thrown over a sleeveless blouse or dress will also work. Capris are fine if they go to at least midcalf. For men at religious sites, have long pants and shirts that cover shoulders and upper arms. But in secular areas, less conservative attire is perfectly fine. In Tel Aviv, almost anything goes! Don’t pack formal clothes unless you know you need them for some reason, say a business function. Israelis tend to dress quite informally, though consciously and with style. (I’m reminded of David Sedaris’s comment that “Americans would be respected more abroad if they didn’t go out looking like they were out to cut the grass.”) I usually pack one dress shirt and one tie, just in case, but never a suit or sport coat.
Even at the Israel Philharmonic Orchestra, I felt just fine in nice pants and a dressier shirt, and some members of the audience dressed far less formally than that! I have never needed more formal clothes at a restaurant, and I often eat at upscale places. A nice shirt and slacks or a skirt or dress will serve you anywhere. Men will need to cover their heads when entering a Jewish holy site, either with a hat or a kippa, also called a skullcap (English) or yarmulke (Yiddish). This is true whether or not the men are Jewish. Cardboard ones are always available, but those are really tacky. You can buy an inexpensive kippa on the street in Jerusalem, so even if you’re not Jewish, pick one up. But a ball cap or other hat is adequate; you simply need to cover your head.