Your First PAX and its your Wife!
I have experience here. I have flown quite a few first-time passengers, including my wife, and have a few pointers I've learned. I have been successful enough that my wife has since earned her license and we are into the fuselage of an RV-7A. In fact, I consulted her to put this response together.
The pre-flight briefing is absolutely the most crucial part. What will cause here anxiety is anything that is unexpected. During the briefing, talk to her about turbulence. Talk to her about what she can expect and feel on a cross-wind landing and takeoff (My wife had one passenger that she forgot to brief on cross-wind landings on a breezy day, and the passenger became scared on final as the plane was crabbing down).
Let her trail as you do your pre-flight, explaining what you are doing (in laymen's terms, please). Take the opportunity to point out the safety features built into aircraft these days (especially the redundancy).
Spend some time with her on the charts and your intended route, so that she can follow along and stay interested. Show her some basics about the GPS, especially the zoom in and out, so that she can verify the position on the charts and play a bit with the electronics.
Ask her if she would read the checklists (preflight, taxi, landing, etc.) as you perform them, to help you out. I am very capable of handling this myself, as a pilot, but it sure made it easier for me, and more interesting for my passengers, when they can feel involved in this manner.
Ask her to keep a pad of paper and pen and write certain things down (frequencies, altitudes, etc.). You may not do this now, but it is good practice for your instrument training and is a good practice for VFR. You have lots to worry about up there and it is easy to forget things. Especially if you get handed off to another center, can't make contact, and want to go back to the previous frequency to re-establish contact.
After each exchange on the radio, explain to here what actually is being said and why.
Explain the controls as you are flying, and what each does. Explain how they have to be coordinated. Then let her fly; first with just the stick while you manage the pedals. Then switch. Then let her manage both. She will most likely NOT do very well, but remember your first experiences. Point out only the good things. Tell her she did as well as you did your first time. Point out how difficult it is (it IS), so she doesn't feel like a dummy. Encourage her, telling her you hope to be doing a lot of flying with her and you would like her as a capable backup.
Don't overwhelm her. Feed her information over several flights, as she becomes more comfortable. By the time my wife started flying lessons, she was capable of flying the plane almost to the runway (pattern and all). I think she was capable of actually landing, should I have become incapcitated. However, this took many flights and a lot of explanation as to what is going on. If you feed it to her too fast, she will become discouraged. Even my wife, at times, said "And you think this is FUN?????" (regarding the landing process). It is a busy time in the pattern and lots of multitasking.
Regarding flying at night, I absolutely love it, and so does my wife. It is much more surreal and relaxing. However, don't under-estimate the additional risks involved in flying at night. Our son goes to college at a school that is 3.5 hours driving, or 1.5 hours flying. We go up there every month, and usually fly back at night. This is in northern Minnesota, so the country is pretty barren. For an additional measure of safety, we fly I-35, at least from Duluth to Minneapolis, before heading directly home. This gives us more lights to work by and a potential landing strip in case of engine problems. The alternative, if you have airports that are close to one another, is to fly from airport to airport, at a higher altitude, so that you have options. Flying is all about leaving yourself options and preparing for the worst (while hoping the worst never happens). The few extra minutes reduces (does not eliminate) the inherent risks. Be honest with your wife about the beauty of night flying, but also about the inherent risks (without scaring her).
Buy her the book entitled, The Right Seat, a book for pilot's companions, by Avram Goldstein. Included is a chapter on what to do in case the pilot is incapacitated. In fact, it is worth reading yourself, as it will make you aware of people's "fears" and allow you to put your own checklist together for breifing the other "first-timers" that you take flying in the future.
Last advice. Ask her how much she wants to be involved. Don't try to push more on her than she wants to do, regardless of the reason. Understand that she will be going through a new experience also and that there will be a lot of different feelings she will be experiencing. She will naturally be a bit apprehensive and will most likely be thinking about her trust in you as a pilot. It will be a time to stay in touch with your feminine side, and tuck the male ego away (Come on, all us male pilots have the egos!!!!! My wife says the air is thick of testosterone at our flying club meetings!)
Hope I didn't get too preachy. This is important and you are right to ask for advice. I've heard too many stories of people that are uncomfortable, simply because it was not what they expected, many times because they did not get an adequate briefing.
Good luck. And let us know how it goes!