Kids of all ages love coloring with crayons, markers or colored pencils. But there is no age limit to enjoying the relaxing and creative benefits of coloring a design on paper, So if you'd like to return to this activity now that you're all grown up, you can find intricate designs to color with the click of a mouse.
Look for some Mandalas. Mandalas are intricate geometric patterns that are fun to color. You can find many patterns for free online.
Search for Frank Lloyd Wright designs. His designs, especially his work in stained glass, make excellent coloring pages.
Find intricate patterns on items from nature. Look for sites that feature animals and flowers. The patterns on reptiles, the shapes of flowers and the mosaics of a garden, are all rich sources to draw from.
Do a search for artists known for their compelling designs, such as M.C. Escher and William Morris. You'll get an education in art history, along with some very sophisticated coloring pages.