Hi Ed,
For tours, you really can't do better than Daniel Bergmann. Superb photographer, outstanding local knowledge, great guy and very focussed on giving people what they've paid for (disclaimer - he's a personal friend, but even if he wasn't I'd recommend him).
Otherwise, well, Iceland is bigger than it looks and travel distances are long. For a first visit, my recomendation would be visit in early June, rent a car (you don't need a 4WD for what I'm recommending, although it's a nice to have), and focus on the south & south west.
First, the Reykjanes peninsula immediately around Keflavik airport is often overlooked and well worth a few days. It includes geothermal areas, lava fields, lakes, great cliffs / birds, possibly best whale watching 8can't speak from experience there). Then, on highway 1 heading east you can see two epic waterfalls (Seljandafoss & Skogarfoss), and take in Skaftafell National Park (more waterfalls, old farm houses, glaciers) and Jokulsarlon glacial lagoon. And take a trip out to Ingolfshofdli, where you should see more puffins than you'll ever need, not to mention that airborne menace, the brown skua.
Doubling back on Hwy1, head off and up to Gullfoss (reference epic waterfall...) and Geysir, then finally hop over to Thingvellir for peacefull vistas, a tranquil lake, the continental plate divide, and guess what, yep, another waterfall.
After that, plan your next trip :-)
David