San Ignacio, Belize is a veritable island. In fact, it's local name is Cayo (what the district is named for), which means island in Spanish. It has the Mopan River, coming from Guatemala, on one side and the Macal River, coming from the Maya Mountains, on the other. The two rivers meet at the edge of town and form the Belize river.
The canoeing / kayaking is an all day trip. You'll start in town under the Hawksworth Bridge, where you'll be paddling up the Macal River toward Chaa Creek.
The tubing is usually done along with a trip to Xunantunich. After the ruins you'll put in on the Mopan River and float down to Clarissa Falls in Calla Creek, where you get picked up and brought back into San Ignacio. There are lots of rapids you go through and it is very likely you will fall off your tube (unless you have skills). This is an all day tour.
Cave Tubing is a full day tour. The tubing is done at a place called Caves Branch, where you'll find a series of stunning watery caves. Because of the low celling you are provided with helmets and life jackets. There are no real rapids, so this trip lends itself to a variety of ages.
Barton Creek is a very pretty, very secluded valley on the edge of the Maya Mountains. The creek actually comes out of a cave - The Barton Creek Cave. You can tour the river-cave in a canoe.