I can't imagine to and from Tikal in a day from Caye Caulker. I'm not sure you could realistically do it. Assuming you can catch the water taxi exactly when you needed to it has to be around 5 hours each way.
Xunantunich might be doable bit its probably 3 hours away from Caye Caulker. That might be interesting since you could see some country side.
Here's a link to a Caulker site running tours to ruins. I haven't been to these ruins and haven't gone with Tsunami so this is just information, not a recommendation. http://www.tsunamiadventures.com/mayan%20ruins.htm
We split our trip up between Caye Caulker and San Ignacio but we had 9 days.
If you want to read about that and see the pictures try www.AlandSueJohnson.com
Pick Trip Reports / Belize
Start with Caye Caulker 2005 and then look at San Ignacio. That's the order we did it. There's pictures of Caracol and Tikal.
Pictures of Xunantunich are in the Placencia trip report and pictures.