Define what is fun for you.
Israel has a LOT to offer. If you like museums, every city has them. Notable ones include Yad Vashem (the Holocaust museum) in Jerusalem and Independence Hall in Tel Aviv, but there are various kinds all throughout. Art museums in major city, and Tel Aviv has the White City for Bauhaus architecture as well as a good walking tour of street art if you're interested in that kind of stuff. If you like gardening or flowers, I suggest going to Haifa to see the Baha'i gardens, which are breath-taking even if you don't have that particular interest.
If you are a more outdoorsy type of person, Israel is a good place for you too. Tel Aviv has a great beach, but Eilat also has some options as well. It's supposedly the place to go if you want to snorkel in Israel. There is a lot of great hikes throughout the nation, from the Golan Heights to Masada where there are some notable Roman ruins at the end of your hike. If you like beauty or just want to be at the lowest point on the Earth's surface, then visit the Dead Sea which has the saltiest waters that you pretty much can't sink in.
If you are a religious sort, obviously Jerusalem. The Western Wall is a must, and depending on your religion there are other places to visit. Christians can go to the Via Dolorosa and the Church of the Holy Sepulcher, Muslims can visit the Dome of the Rock, and others can just appreciate these different areas. But there are other religious sites outside of that one city: Safed for mystics, the Jordan River of Baptisms, and many other places. I'm not sure if religious things are "fun" but some people have to include that in their itineraries.
There is also great nightlife in Israel, most notably in Tel Aviv. If that's your scene, then there are a lot of cool bars and clubs to go to. Israel also has great food, even the street food tends to be very good. And if you like shopping, you can do some higher class stuff in the cities or try bargaining in the open air markets (shuks).
There are a lot of fun things to do, so you may have to be specific to get a more personalized answer.