Well...prior to falling in love with DLR during the Christmas season or becoming enchanted with Halloweentime, I used to go to DLR at "regular" times of year (summer, spring - that sort of thing). If I were still going at regular times of year, what I do when I first enter the gates might be different than what I do during the holiday season. So I will base my answer on the premise that we are talking about the holiday season and your upcoming Christmas trip.
So the first thing I do when I get into DL is head to the gigantic Christmas tree on Main Street for PhotoPass pictures. Actually, if there is a PhotoPass person by the floral Mickey in the front, I will stop there first and then head to the tree. If it happens to be a Magic Morning during Christmastime, I will high-tail it all the way back to the far corner of Fantasyland to get on It's a Small World Holiday before the line gets crazy. Even if it's not a MM, that will be my first stop. While I love to look at IASW Holiday at night, when the breathtaking colored lights come on, I prefer to ride it in the daytime when the lines are not as bad. Something happens at night when they turn on the lights - the colors are so gorgeous in person that they literally seem to attract people from all corners of both parks like moths to a flame, and the area around IASW Holiday at night is absolutely jam-packed with bodies.
So that's my tip - get on IASW Holiday as early in the morning as you can and experience the ride itself, and then, come nighttime, saunter back by there after dark to soak in the beauty and see the light show that is projected onto the facade.
If I am going into DCA first, for some reason, I high-tail it back to the Paradise Pier area and get on Toy Story Mania before that line gets nutty, stopping for PhotoPass pictures at the DCA tree along the way.
And if we happen to be eating dinner or checking into our hotel first, before entering either park, I will head to the GCH lobby, find a comfy chair or sofa to plunk down in, and listen to the carolers when they come in and perform, watch people get their photos taken with Santa by the giant GCH tree, and soak in the overall Christmasy atmosphere (preferably while munching on a gingerbread man w/ Mouse ears cookie

)!!