i've been using centers in my k-3 classroom for about 12 years, and each year i've tried some new centers and eliminated those that sparked little interest...
each center is large enough for 4 students to work in, and they are the students' main activity for the day, not a reward for finishing early... they choose where they work each week to express the theme of the lesson... when they've completed 9 different centers they can repeat their favorite center for the rest of the year as long as there's room for them when they are called to choose...
the tried and true centers are...
building blocks on a 9x12 rug
sand sculpture in 4 2x3 landscape tubs
paint easels with dried tempera blocks
light tables with color pencils
chalk boards arranged easel style
computers - with open eyes, artus the mouse, and kidpix4
crayon melt on hot plates set to just below the 'w' on the dial
i had an article in schoolarts a few years ago with the second part of the article online... look in the back issues... i think it was a march or april issue... it showed a basic floorplan for my classroom that year... each year i like to change it around a bit...