While I haven't been myself, I've done some research so figured I may as well share.
For tours and excursions:
The Siwa Guide - desert safaris, and they describe a sea of sand stretching as far as the eye can see. They have half day, full day and custom trips - which could include camel safaris, or camping under the stars.
Abu-Zahra Bedouin Safaris - promote themselves as more environmentally-focused tourism.
As for accommodation, you haven't really specified budget, medium or high:
For budget - Palm Trees Hotel sounds the most fun and scenic, with "nice garden with trees and fire pit, where the staff will sit at night with guests. Roof terrace with a view of the oasis and Shali" (review on Wikitravel)
For medium - Shali Lodge - sounds pretty smart for good value, and has some exotic local foods on the menu - date omelet, lamb casserole or vegetarian tajin, each flavored with fresh herbs of the oasis.
For deluxe - two amazing and different options - the Adrere Amellal: Desert Ecolodge - with no electricity, the rooms are softly lit with a dozen beeswax candles, and around the corner, the Tamazigh, a smaller hotel, with a spring-fed pool and some rooms made entirely of salt!