I personally think it's a shame to visit Belize and not spend some time on the mainland (unless you're coming back soon and can concentrate on just one area).
San Ignacio itself is a pleasant little town of 13,000 or so people. Don't expect too much, but it's nice for a day or two. And there's really a lot to do around San Ignacio -- caving, hiking, rivering, visiting the Pine Ridge, seeing the Zoo and butterfly farms, horseback riding, etc.
Keep in mind that not all jungle lodges are in "real" jungle. Many of those in cottage country around Cayo are in or near cleared land, citrus farms and ranchland.
For more of a jungle experience, think about places like Ek 'Tun, Black Rock, Jaguar Paw, and Pook's Hill.
Belize First http://www.turq.com/belizefirst/