In order for it to be a surprise, you will need to have your parents gone for the evening. Maybe you can drop a subtle hint that your dad should take your mom out for dinner or something. The night when we did our surprise, Dad took Mom out, so we had the evening to prepare.
The idea is to make their bedroom look like a room at a hotel or a bed-and-breakfast. We did the bed-and-breakfast. First, we cleared out any clutter that was hanging around and moved it into the spare bedroom. Then we put a pitcher of (silk) flowers on their dresser. In the bathroom, we folded two sets of towels the way they do at hotels: Take the largest towel (the bath towel) and fold it in thirds; then fold it in thirds again, with the edge inside (see illustration). Set it on the bathroom counter (if it isn't firm enough to stand by itself, you can fold it in half again). Now take a hand towel (not the washcloth), and fold it in thirds, as you did in the first step of the bath towel. With the edge down, lay it over the top of the bath towel, lengthwise. Now take the end that is on the outside, and partially open it up. Fold the bottom up, and tuck the sides around the back, forming a pocket. Now take the washcloth, "accordion" fold it, then pinch it in the middle and fold in half. Stick it in the pocket so that it fans out of the top. Voila! (Now, did that make ANY sense??).
After that, if you have any miniature shampoo and conditioner bottles, maybe from those little bags they give you at the airport if you're flying overnight, put those on the counter in front of the towels too, or in the shower. Another nice touch would be to put a still-in-the-wrapper bar of soap on the counter and/or in the shower.
For the actual bedroom, you can create some ambiance by lighting a few candles and putting on some quiet, classical music. If you happen to have live flowers handy, especially roses, you can sprinkle some of the petals on their bed. Be sure to dim the lights!
My sister and I wanted to put a chocolate on each of our parents' pillows, because they do that in high-end hotels. Alas, we didn't have any in the house! Not being easily daunted, however, we melted some chocolate chips, poured it into 2 heart-shaped molds, and put them in the refrigerator to harden. When they were done, we wrapped them in foil and put them on the pillows.
For the final touch, I made up a brochure to look like a bed-and-breakfast brochure. We just moved to our new house, and were considering names for the house. One of them was Fairview, so I made the name of the B & B the "Fairview Bed & Breakfast". The brochure is folded in thirds. Below is a sample of the brochure I made. When making your own, make up creative names for the different rooms and "amenities". For instance, at our house, the kitchen table, where we generally eat breakfast, has a deck outside the window and there are always chipmunks running around out there; our cats go crazy watching them! So I called this "The Chipmunk Ridge Breakfast Parlor" and I assure you, my parents got a kick out of it! The "Conservatory" is really a small greenhouse, "The Beach at Glacier's Mirror" is our pond. Do you get the idea? Make it sound like a resort, while at the same time giving it a humorous twist. Click on the images to enlarge.
Well, I hope you have fun creating a special evening for your Mom and Dad!