Sometimes. When I was a teenager, I would overpack because I want to make sure I didn't run out of clothes!

ASN: Drive with the windows down?

~Wunder

Sometimes! (I do like the wind though)

ASN: Think of something funny at the worst times... and then struggle not to laugh?

Sometimes! Oooh, I could share stories.

Stay up very, very late to finish a certain section of a book you're reading, even if it means you may be really tired the following day?

Sometimes! When I was younger, I was the one getting in trouble for reading with a flashlight with blankets over my head!

ASN: Procrastinate?

~Wunder

Sometimes, especially when it is something I am not really happy doing.

ASN: Make dreadful mistakes?

Always!... OK, maybe just sometimes. After all I did join Narniaweb!

ASN: Get told to dress up in a costume for an event... and then ponder how to work something Narnian into it?

Never. I don't think I've ever gotten told to dress up in a costume for an event, hehehe.

ASN: Swim at the beach?

~Wunder

Always if you mean any time I go to the beach, I swim. If you mean how often in my regular life I go to the beach... sometimes.

ASN: Borrow books at the library even though you already have a bunch to read?

Sometimes. I am firm believer that one can never have too many books to read.

ASN: Make your bed?

~Wunder

Always- unless it's sheet-changing day. Then I tend to just strip my bed and put off actually making it.

ASN: Sit on the floor when there's chairs available?

Sometimes. Usually, I sit on the floor if the chair I'm sitting in becomes uncomfortable.

ASN: Dip your fries in ketchup?

~Wunder

Sometimes. Unless there's a better dipping material available.

ASN: Find something you wrote a while ago and can't understand your own handwriting?

Sometimes. Usually, if I wrote in a hurry or in a small space.

ASN: Put sugar in your coffee?

~Wunder

Always. I drink tea without sugar but coffee has to be sweet.

ASN: Take the train instead of driving?

When having to head into busy Toronto (a couple of hours away): pretty much Always.
When I need to get someplace around here (a quieter, more rural area): Never (no trains from here to town )

Going off Wunderkind's post above:
Sprinkle white vinegar over your french fries? ('Tis a very Canadian thing, so I would be surprised if anyone else does, ha.)