My friend just drove me up to Malacca over the weekend as well. We went in on Friday afternoon, reached Malacca at around 7pm, and then came back on Sunday, reached SG at around 2+pm.
This was actually my first road trip, and we didn't have any scary encounters on the road, that's for sure. We chose rather busy petrol stations to stop by for refuel and toilet breaks. The people we met at those petrol stations are generally very friendly, and oddly I don't feel uneasy or uncomfortable when I was there. Just be alert and you'll be fine.
We only used the turn-by-turn navigation from Google Maps in our phones, and it worked just fine.
If you are driving and you see some cars from the other direction kept flickering their headlights, most likely there's police up ahead. Just slow down (if you are hitting the accelerator hard) if you don't want to give kopi lui. I thought this "secret highway code" of NSH was a myth/joke, but it really did happened on our way back.
Oh, also, in case you intended to drive to the night market, Jonker street is closed from 6pm to 12am on Fridays and Saturdays to make way for the night market.
There are a lot of private parking lots in Malacca, where you need to pay the attendant a small fee to park. It can range from RM3 to RM5 per lot. I was told by the hotel staff that after 6pm, you can park by the road sides without the need of a parking coupon. That was what we did once, and we didn't get any fines or ticket.
You can get around Malacca by walking, but the afternoon sun is really very very scorching. If you intend to walk to somewhere far during the day, it's better if you can drive. My friend and I walked from Jonker Street (where our hotel was) to Mahkota Parade, and we regretted not taking the car because it was far too damn hot. (It didn't help that we tried to make a side trip after that to Pak Putra, only to find the shop closed till early March =_=.)
If you or your wife likes crepe cake, do give Nadeje a try. Their crepe cakes are lovely, and is easily 10x better than the Lady M's in Singapore.