My sister loves theme parks and thrill rides (not my
cup of tea), although I've been wanting to visit Disneyland
since a was a little kid.

There's a wide variety of things to do at Disney, so you and your
sister will both have fun. I'd strongly recommend studying a good
Disney guidebook before your trip, so you can make the best use of your
time. There are also some Disney newsgroups and websites where you can
get advice.

When in Vegas, your sister might enjoy the rides on top of the
Stratosphere hotel.

What is usually the best time of the
year to visit CA and Vegas?

The weather is fairly consistent year-round in California, but the
weather is cooler in the northern part of the state, and also at higher
altitudes. At Yahoo Weather (and similar sites) you can find averages
for any particular city. Just find the city, then click the "Averages"
link. For example, here are the averages for L.A...

http://weather.yahoo.com/climo/USCA0638_f.html

Las Vegas has a very different climate, with the summers much hotter
than the winters...

http://weather.yahoo.com/climo/USNV0049_f.html

The Grand Canyon will be cold and snowy in January, and trails may be
icy in places. Some enjoy the Canyon in that weather; others prefer to
visit when its warmer. Check the official site for more info.

James