Saturnalia falls at the time when non-Romans are celebrating Christmas, Hanukkah, Solstice and/or Kwanzaa.
Saturnalia festivities began with ritual and sacrifices in the Temple of Saturn. The statue of the god was hollow and filled with olive oil, as a symbol of his agricultural functions. His feet were bound with woolen strips, that were unbound at Saturnalia.
After the rituals, the Senators (who had to be present) dismissed the crowd with the cry of "Io, Saturnalia!", a sign for the happy festivities of family parties and other private gatherings to begin. The traditional gifts were wax tapers and little dolls, although gifts of silver later became traditional.
The custom of the Lord of Misrule was appropriated and survived through to English Christmas traditions.
Roman decorations and celebrations Secunda Floria Zonara: Many of the decorations involved greenery - swathes, garlands, wreaths, etc - being hung over doorways and windows, and ornamenting stairs. Ornaments in the trees included sun symbols, stars, and faces of the God Janus. Trees were not brought indoors (the Germans started that tradition), but decorated where they grew.
Food was also a primary decoration - gilded cakes in a variety of shapes were quite popular, and children and birds vied for the privilege of denuding the trees of their treats. The commonest shapes were fertility symbols, suns and moons and stars, baby shapes, and herd animal shapes (although, to be honest, it's hard to tell if some of those ancient cookie cutters are supposed to be goats or deer). I would imagine coins were also a popular decoration/gift.
People were just as likely to be ornamented as the trees. Wearing greenery and jewelry of a sacred nature was apparently common, based on descriptions, drawings, and the like like from the era. Although the emphasis was on Saturn, Sol Invictus got a fair share of the revelry as well.
The biggest part of Saturnalia was attitude more than decoration. Feasting, drunkenness, merrymaking, hopefully the conception of more children (or at least enjoying those activities which led to conception!), pranks, gift giving, role reversals (not true ones, only symbolic ones - slaves weren't really free to make a freedman's decisions and anything they did or decreed would reverse at the end of Saturnalia, children weren't really adults and could not enter into any binding contracts or make business deals, etc.) and so forth.
From Saturnalia Practices in Nova Roma http://www.novaroma.org/religio_romana/saturnalia.html