We did/do a lot of the traditions sans mythical creatures. We color and hunt Easter eggs (all three teenagers still like participating along with their younger brother) but they know it's dad and I who are out hiding the eggs and they get baskets and/or candy. I don't put the tooth fairy money under the pillow, but I do give them money for lost teeth. They have generally been pretty happy with the arrangement as far as we can tell.
I did actually find it pretty disappointing to discover there was no Santa and felt deceived, but it was in a greater context of deception.
If one of my kids asks about something I try to explain it the best I can and if I think they need to know about stuff, I tell them. Too much information can be as bad as too little/incorrect information. If I don't know the answer to something, I tell them that, too. And sometimes I tell them several different schools of thought if it's something based on opinion.
I'm also dealing with autism with my sons. I know they are capable of blurting things out at inopportune times, so I take that into consideration. I also take into consideration that they often see things in a very black and white way, so the relative value of truth is not always there -- a lie is a lie is a lie to them. There aren't little and big lies or malicious/harmless/polite lies . . . yet. And I spend time explaining that while we may not think something is important, people have to be humored to some degree and explaining/translating social concept that other kids often grasp intuitively.