Notice that the ground rule was no vandalism. This means that nothing would be done that would involve more than having the Witlesses having to throw something away.
If you wish to spend only $10 or so, it is quite hard to do. I have found that, at Target, 20 feet of red Christmas garland strung up around the front alcove or doorway is only $3 (at Wal-Mart, you can get some lower-quality garland 60 feet for $5, which I recommend seeing where it is likely to end up getting thrown in the trash). If you have a brown or dark color wall, silver garland works best. A lighter colored entrance, or an aluminum one, goes well with red. Putting it up where it can reflect the parking lot lights (without creating an additional fire or electrical hazard) adds to the effect.
If you are willing to spend about $20-25, you might wish to add a wreath. It needs to be at least 3 feet across and have bright red colors on it to stick out like a sore thumb from the road. Adding lights is going to add about $5-10, depending on how elaborate you want them to be (including the cost of the batteries needed to run it). Solar power lights run you about $10 and up. Again, bear in mind that they are likely to end up being put in the garbage dumpster, so durability is not an issue (I highly suggest sacrificing durability for this reason).
A really cheap idea is to find some white trash bags. Stuff them with leaves. I recommend three, the bottom one being at least 2 feet across. Attach them to each other, and add decorations. These can be simple, or even drawn on with a magic marker. This creates your snowman. Total cost is probably going to be under $3 for this, and even less if you stuff them with leaves and draw the decorations on the bags and use sticks (it can get well under a dollar). No damage will be done with this.
One other suggestion that is going to cost you about $4 or 5, is to replace some of the parking lot light bulbs with red and green bulbs or put red and green lenses on the lights. Combined with the snowman and the garland, this can provide a neat effect. Having the lenses right under the entranceway replaced with red and green lenses or bulbs plus the garland is going to create a Christmas atmosphere that the Witlesses are not used to. Just bear in mind that you will have to put the original lenses or light bulbs where they can easily be found so you will not be vandalizing anything.
A word to anyone that thinks this is offensive: The Witlesses did not tell me the truth up front when getting me in. Then, once I joined under false premises, they did not let me just walk out without having to sneak out. This is worse when children are born into it. They are never allowed to walk out and keep their family. They are pressured to get baptized well before 18, and sometimes even before age 10 (I have seen showcased examples insinuating that people should be baptized at age 6). They did not have any freedom to practice what they want. If they try to practice what they truly believe, they are going to get censored in ways that are far more damaging than any of these ideas for setting prank Christmas decorations (which will probably see the inside of the garbage can well before the boasting sessions).