Well, when planning a trip when one hopes to see rare wildlife, I'd ask around (as you're doing here) in several places to assess the probability of actually seeing what you hope to see. You might ask at the Lonely Planet
thorntree for instance.
My trip report described something about equivalent to Chinese hard seat travel. You can certainly do everything soft seat for just a bit more money.
As far as sites, Luang Prabang, for example, is nice, which you have on your itinerary. Packed with tourists lounging over a drink at Villa Santi or L'Elephant, but nice. No ruins, but a several fine temples and natural beauty.
The
falls outside of town are not to be missed. The weaving village is ok. The night market featuring Hmong crafts is nice. The President's Palace. Trips on the river. Monks processing in the morning. Lots of tourists drinking Beerlao or heads-down at the Internet cafes. Phuxi hill. Have a Lao skirt, a
sin made up in the market.
But, if you like ruins, Wat Phu outside Pakxe, also in Lao (see Google Earth, Lat 14.848486°; Lon 105.824246°) is really fabulous with none of the Angkor hordes. Same for Preah Vihear (14.397575°, 104.680478°). Phnom Rung (14.531832°, 102.940204°), while more accessible and more touristed, chiefly by Thai people, is more restored and lets one visualize things as they might have been. And, these are all axial temples, of a kind that aren't represented at the Angkor sites. Muang Tam is also dissimilar to anything you'll see at Angkor.
Another thought would be to fly on Air Mandalay from Chiang Mai to Mandalay and thence to Bagan.
Of course, it's your trip, not mine and you should emphasize the things that matter to you.
As far as getting around in the parks, pick up the relevant Lonely Planet books. They get slammed a lot, but the alternatives for Thailand aren't that much better (except for the South-East Asia Graphic Guide by Mark Elliott, ISBN 1 873756 67 4).