welcome to Cruise-Chat.com.
Your travel agent should have asked you about your sitting preference. I recommend getting in touch with him / her in order to have your sitting checked with the cruise line (and to have it changed, if necessary). You can also look your assigned sitting up yourself by viewing your reservation on the Royal Caribbean web site. You will need your reservation number for that.
As for activities, it is often not necessary to sign up for those in advance.
PS: Hey Dave, I guess you were faster by a couple of seconds!