Many retailers are offering basic cotton button-downs, so as long as you avoid the sheerer white ones, you have several colors to work with this summer. If they're not long enough to cover the thighs, you can wear them with a loose skirt.
Chicago-based Al-anika.com is one of many online retailers which has many great summer-weight long tops in basic colors and some very reasonable prices. For tops, the WeLoveHijab blog has even suggested looking at beach cover-ups, saying that "while most of us won't be shopping for bikinis…we can still head to the swimsuit section for other reasons – swimsuit cover-ups. When it comes to summer Hijab fashion, swimsuit cover-ups can be a Hijabi's best friend. Many of them come in the form of long-sleeved tunics that Muslim women can wear comfortably all summer long."
Linen or cotton-blend pants are great for feeling cool when worn with a long shirt. Wide-leg pants are a great way to feel light without the hassle of a skirt, especially on breezy days when skirts billow around too much. Such pants can be found in dressy styles as well as more casual ones for just lounging around at home.
Coupling long cotton t-shirts with a very lightweight long open cardigan will give endless color options at a very cheap price, while being breathable and comfortable for summer. Some people will visit the men's department for the shirts to get a longer length.
The combo works equally well over a contrasting linen skirt or over matching pants. Just play around with your options. Try mixing a solid lightweight polyester scarf and solid cardigan with a printed t-shirt, or a solid t-shirt and cardigan with a printed scarf.
As far as scarves go, most are made of lightweight polyester. Cotton ones tend to be limited in choice of color and design, but do keep those for when you'll be out in the sun a lot. Wearing a light-colored Al-Amira Hijab is a great option for summer if you're going to be outdoors, and there are many more color and designs available. Just remember to drape your chest with a very lightweight Shayla though. You can do this by wearing it across or over your shoulders.
Outside or for the home, also consider the long sundresses that are in season. If they're sleeveless, take a short cotton blouse and wear it open over the dress to cover your arms. Or, buy a long-sleeved t-shirt in a size too large so it's not too tight, and wear it under the dress.
This summer, relax. Keep it casual, simple, and lightweight.