Seminyak and Kuta are the two tourist capitals of Bali. Seminyak is about 15-20 minutes North of Kuta, but they are both located on the same stretch of beach. These locations are ideal if you like surfing, shopping, eating, and partying.
Tanah Lot is a temple located right outside of Seminyak. You can hire a driver to take you there or rent a scooter. Apparently there are beautiful sunrises and sunsets at the temple, but no matter what time of day you go, you'll enjoy the sights. Tanah Lot can get extremely crowded and there are vendors everywhere trying to sell you things.
The main attraction of both Kuta and Seminyak is the beach. It's a wide expanse of surf and sunbathing. You can rent a chair and an umbrella for a day, or take your towel and sit on the sand. Either way, make sure to apply and reapply sunscreen. Surfing is another crowd favorite. Even if you have never gone before, you can get lessons for about $10 USD. The surf is big enough to learn on, but small enough where you won't be intimidated. No wonder Bali is one of the surf capitals of the world.
It was wicked stormy when we went, but it didn't stop us from enjoying our time. If it starts raining, duck into a bar or restaurant and wait it out. Storms only last an hour or two! Another option would be to get a massage or manicure. For only $7.50 USD you can get an hour massage - that's not a bad deal!
So, what's the difference between Seminyak and Kuta? They're extremely similar. Seminyak may be slightly more upscale and nicer. Kuta has a bit more of a party scene. What we did is spend 2 days at each location. I prefer Seminyak, but both are wonderful and you can easily have a great time at either!