Springs Preserve Tour:

Tours last 1.5 hours, and cost 15 dollars a head.  Afterwards, visitors can explore the grounds or museums at no extra charge.  Hours of operation are 10-6, tours can begin by 9.  Tour options are: site overview, wildlife, garden, archeaology, and sustainability.  The sustainability tour explains the 'green' technology behind the Desert Living Center.  Reservations need to be made at least 2 weeks in advance.   Tour groups must be at least 10 people and no more than 25.

Hoover Dam tour:

The guided portion of the tour is a 45 minute trip 500 feet down to the power plant inside the dam.  Otherwise, the cost includes access to a movie and exhibits inside the visitor center and walking around the top of the dam (of course).  The guided tour can apparently accommodate 80 people (and maybe more) at a time.  Reservations can be made beginning 90 days prior the tour at no charge – until shortly before the trip: they would like a head count and payment 2-3 weeks before tour for a big group, or within 2 days ('or whatever works for you') for a small group (say, about 20 or less).  The cost as of now is $11/person, and that includes parking.  Tours start every 15 minutes beginning 9 am (until 4:15 pm).  Given the large number of people allowed per group, I was lead to understand that even if we come when it is crowded, the wait should not be more than 15 minutes until we can begin the guided part of the tour.  Presumably people can explore the visitor center exhibits or see the movie during any such wait time.