Surprise trips to DL are a killer, I can't keep my mouth closed! Just keep reminding yourself about the looks on their faces when they realize where they are going. I tried to keep it a secret from my DDs but as my SIL is coming in from Denver and is going with us (this is a girls only trip as my DH can't make it due to work), I couldn't figure out how to explain why she was in town in the middle of the week. I also wanted then to understand a little bit of how much a trip cost, so they are making some finacial sacrifices. One of the things they are giving up is dinner out each week (Friday night is family dinner out night so Mom has to make a sacrifice, too, and actually cook on Friday night, boo hoo! Oh well, if it gets me to DL then I think I will survive). We save what we would spend on dinner and put in a DL fund. This goes for meals and souvenirs. They will each get a portion as spending money of their own to spend any way thay wish.
Back to the surprise, just be quiet and smile a lot and pack after they go to bed the night before you leave. Maybe I will tell my girls that we can't go and then surprise them when their Aunt arrives. Good luck with your surprise.