Arenal area of Costa Rica

There have been several Costa Rica threads on here, but for some reason I am unable to find them now. My wife turns 30 in January and I am trying to plan a surprise trip to the Arenal area of Costa Rica. We should have 4 days for activities while we are there. I have read about the area online, but the amount of information is hard to sift through. Do we need to rent a 4x4 or take the shuttle from the airport? Where is the best/most economical place to stay? Any things to stay away from? Must have items? Best tours? Wakeworlders always have pretty good advice on a range of topics, so let hear it!