Having the sun out can be a blessing or a curse depending upon how the photographer is able to utilize it. Many believe a cloudy day provides the best lighting for photos, especially outdoor portraits. However there are some very gorgeous effects you can accomplish on a sunny day, one example is backlighting. Backlighting can provide a soft and appealing glow to your subject, however exposure can become an issue. If the subject/model is not properly exposed then you can have a blown out background with a somewhat properly exposed face, or a a face covered in shadows even when you do expose the sky properly. There are a couple ways to approach this, with and without a flash, and artistic options as well. Sometimes you want to expose for the background or sky and have a dark subject, an example is the photo below.
I chose to expose the beautiful colors of the sunset and use the subject as a silhouette. This leaves a little bit of mystery to the photo but still conveys the feeling of the moment. In this image you can also see that the backlighting (glow around the shoulders) helps to separate the subject from the background so the image does not appear flat. In order to capture this I added a neutral density filter (ND) in order to essentially put sunglasses on my lens. This allowed me to capture the light I wanted in a more pleasing way, as it was actually quite bright outside still. Should I have wanted to expose my subject, even using these same settings, I could have used a flash to pop on the subject and bring her out. This would have allowed me to still maintain the colors of the beautiful sunset, but also expose the subject so you could see who it is. It was simply a stylistic choice.
Using a fill flash is not the only option to properly expose in bright conditions, but it is one way. In order to do this you would simply want to put your exposure metering on "spot metering" (looks at very center 2-4% of frame) or "center-weighted metering" (looks at larger area of center frame) mode and then you would aim your center focus point in your viewfinder on the sky or area you would like to expose and set your ISO, f-stop, and shutter speed to properly catch the highlights and colors of the scene. You would then re-compose your shot (frame the picture in the way you want to shoot it) and use a flash to fire on your subject that you want to highlight.
In the image to the left I wanted the background to not be blown out (too highlighted with no detail), so I exposed for the background and then used a very soft and low powered flash to fill in their faces to avoid shadows. The sun was low at this point and was creating some shadows I was not happy with. Another thing about a cloudy day is that the light is diffused well but is diffused vertically, meaning it is coming from above. This can create dark shadows under the eyes even with the beautiful soft light you get. A low powered, soft fill flash can correct this. You can also use your environment to avoid harsh shadows on a sunny day, while still utilizing the beautiful light that can exist throughout the day.
If you do not have or do not want to use a flash, there are still ways to accomplish beautiful portrait photos on a sunny day. First off you can still utilize the sun as a backlight by placing your subject between you and the sun, so the sun is at his/her back. You can then use a diffuser to help soften some of the light across the face, or you can use a reflector to bounce some of that back sunlight onto the subject's face. Moving the subject under a canopy or into the shade can help even the light, and then you must expose for the subject's face using spot metering again and setting your ISO, f-stop, and shutter speed settings appropriately. A quick note, you can use manual mode to do this and your settings will only change when you change them. However, if you are using AV, TV, P, or other assisted modes then you can use AE Lock. Most DSLRs, if not all, have a button that says AE-L or AE-L/AE-F. If you are in a creative mode you can expose for the face or background and then hit the AE-Lock button to lock your exposure, re-compose your shot, then fire away!
You can also use the bright light to your advantage. In the photo to the right I purposely composed the shot so that I would get light flare, to add an artistic touch to this photo. I also exposed for the subject/model and allowed the background to be blown out, because the blown out background adds to the artistic style I was looking for. This shot could have been completely different had I chose to do a typical portrait. I would have cropped closer on the face and had her looking forward. I would have used a hood on my lens to block lens flare and I would have exposed to capture the details of the sky. Then I would have used one of the techniques discussed above, such as a fill flash, reflector, or just properly adjusting my settings to expose using natural light (if possible).
Bottom line, there are many shots that require a sunny day, and even some harsh lighting. Cloudy days are great for getting even and soft light diffused through the clouds. However, sunny days do not mean you can not go outside and get some great portraits and people shots. Enjoy the sunshine, get creative, and have fun!