The right theme for the party can affect your guest's perception and attitude from the moment they are invited and, as stated above, does not need to be a holiday, birthday, graduation, shower or other sort of celebration, but just a reason to get-together.
With such "just because get-togethers" the theme need not be anything too extravagant. Since food is always needed at parties - dinners / brunches, or simpler barbeque / crawfish boils can provide you and your guests with something just as nice to look forward to and, as such, the food, itself, can be the theme. However, this does not need to be the case; simple finger foods such as sandwiches, chips and dips, pizza or even a pot-luck in which each guest is asked to bring a covered dish (whether it be a dessert, entr�e or side dish) can keep guests happy while other activities are planned to help spark new insights and discussions towards keeping things interesting.
For small groups - gaming parties can be an easy way of getting people together for some fun, and whether it's a card, board, or video game, make sure all the people you invite can join in. Movie parties, in which videos, DVD's, etc. are shown, can be good if you know everyone's movie taste, or if you wish to expose your guests to something new that you think they'll enjoy. Of course, live entertainment (depending on the entertainer and the sort of entertainment you select) can present something fun, exciting and / or thought-provoking for your guests to talk about for a long time to come. These activities can become the theme.