Two Problems

I don't often post my parenting issues here, but I'm really stuck with these two things, and they're driving me more than a little batty.

1) Towels. How do I get my children to hang up towels? It has been weeks, nay months, where every time they swim or shower, the towels end up on the floor. Either in the bathroom or the bedrooms, but always on the floor. Even when it's a towel for drying hands or after toothbrushing, 95% of the time it ends up on the floor. I don't get it. I literally spend the 5 minutes every night picking up towels from both bedrooms and the bathroom, and put them back on the towel racks or over the shower rail. Obviously it's not a time issue.
What can I do to get it through their thick little heads that hand towels don't even need to come off the rack?? And that shower/swim towels NEED to be hung up in the bathroom? I admit the shower bar is too high for most of the kids, but Katherine can fling it over. And the towel bar by the sink is low enough for even Jonathon to put the towel back.
I got so annoyed today that I removed every towel from their bathroom and left them a roll of paper towels. It's SUCH a waste though. What else can I do? They don't even seem to see the towels though they are literally walking on a carpet of them.
2) Dinner time. I don't know where my kids got it in their heads that during dinner they can get up and walk around, play charades, crawl under the table, etc. Nicholas is the worst offender. I (again) got so fed up today that after he got up and literally started walking in circles by the kitchen door I told him to sit down and if he got up again he'd be eating his dinner my himself the next 2 nights. Not a minute later, he slid off his chair again.
I don't get it. We've never changed the rules, yet somehow this has creeped up on me and now it's ridiculous. Could it be because I let them get up to get their own stuff (i.e. condiments they want, ice, clean fork)?
How can I get them to sit at the table and then stay sitting?