I don't think anyone needs an excuse for a party in the summer--just celebrate good weather and good friends!
One fun summer party theme is "Tiki" or "Luau." Party stores stock up on these types of party decorations each summer.
Party ideas can be found in books such as The Party Girl Cookbook by Nina Lesowitiz [642.4 LES]. And, to make it extra special try a few Tiki bar drinks.
Check out this webpage that has recipes with wild names such as "Fog Cutter" and "Navy Grog." [By the way, the page is from Retro Planet: Out of this World Retro Gifts, Decor, Furniture & Vintage Signs!, which just so happens to be based in Nashua--click here for more info.]
Other drinks recipes can be found in 101 Sangrias & Pitcher Drinks by Kim Haasarud [641.874 HAA] and Coolers and Summer Cocktails by Elsa Petersen-Schepelern [641.2 PET].