We do both. Like you, when we had one child, it was certainly easier to drop everything and run off- and we did so all the time; not so much with three!

However, my kids are all older teens now and I've found that as the kids get older we're able to be fairly spontaneous again- providing they don't have anything planned for the night/weekend, etc.

It also helps that I taught them to be responsible for packing their own gear when they were small and that we store all of our camping gear in one or two large plastic tubs. If we want to take off for a bit we just need to throw the tubs in our vehicle and away we go.   

It really depends on the purpose of the trip for us. We do a combination of both. If we have a deadline of somewhere we need to be, it is planned. If no deadline, we just go, and get there whenever.

We do both, also! Just depends on our mood. We have a few planned trips scheduled ahead of time throughout the year, but we always make take some spur of the moment trips during the summer months. The only thing I can't do un-planned now is winter camping. I need to think ahead and make sure we have everything we need with a toddler in tow!