Lets see
For dds first b-day we bought her a tiarra (sp? ya know, little crown thingy) that she will be able to wear for many more years. I think it was about $3. They have them at Claires. Had probably 5 different ones. They are also easy to make. It was kinda gaudy but she looked adorable in it. Another stor had some made out of regular cardboard cone hats, but they had glued Eyelette around them and some ostrich boa feathers around the bottom and right on top. Used slick pen to write "Birthday Girl" on it and were charging $25 for it. I should make those. Obviously something not meant to be thrown away. You could also have the kids make a decorate thier own hats. This provides an activity as well as a take home item which kids love.
Instead of party hats you can make flower garlands (not reusable but lovely and natural oh, unless you used silk flowers) or you can use some wire etc and make simple head wreaths (we use the kind you find at Christmas with stars and snowflakes and such and use about three strands of it, twist in back to secure) and add curling ribbon. My children have had thiers for over a year and keep them in thier dress up box (hey these could also double as favors

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Pottery Barn has personalized (or not) canvas birthday banners. very nice yet festive. Also check stores that sell garden flags, they usually have birthday ones which could be hung on the wall or on a flag pole.
Flowers always make nice decorations. Ribbon is rarely used more than once regardless of what type it is but it easly dresses up just about anything. Most nice ribbon can be ironed and used again.
Festive dishes can usually be found cheaply at discount store and reused for years after. You can buy colorful placemats and table linin that you use for party's for everyone in your family or to celebrate any special day.
If you wanted to fo a theme and get disposable stuff, a way to minimize the impact is to treat it gently and reuse it for another child or trade it here at MDC. Most of that stuff (except for paper plates, cups and napkins) could easily last through 5 or 6 b-day parties. My friend made a killing off all her left over reusable stuff at her last garage sale.
beach balls make a fun alternative to ballons and are cheap enough that you can send them home with guest.
As a gfeneral rule my kids enjoy the host more than the decorations or themes. At dds last party the pinyata (which I made) was truely the big hit. Such visciousness for such little girls. Bubbles and side walk chalk are great summer activities, dress up tea partys are always a hit ( you could use the clothes and some mirrors as decoration and set up a photo lab and take some poleroids) It is easier to come up with decorations (reusable or not ) when you have a theme or a decided non-theme.