San Dimas is actually the western most city in the Inland Empire. It actually could also be considered the eastern most city in the San Gabriel Valley. Surrounding cities are Glendora to the west, La Verne to the east, with Walnut and Pomona to the south (LA county owned property is to the north).
There are many things to do in San Dimas. Probably the most well known attraction is Raging Waters, as mentioned by fhayashi. Bonelli Park, which houses Puddingstone reservoir, is also known in this town. I would recommend against swimming in this lake, since it is quite polluted. Although, it still remains a very beautiful area, and its still fun to fish there.
This town is in a perfect location. Its 30 minutes to LAX, 15 minutes to ONT, 35 minutes to any beach, 40 minutes to the high desert, 20 minutes to Disneyland, and 35 minutes to Magic Mountain. The 210, 30, 57, 10, and 71 freeways all come to a close there.
Formerly known as "Mud Flats," as noted by the Jedediah Smith monument, San Dimas is a "western" town. Montana's (formerly Western Connection) has live country music on the weekends, and Boot Barn (formerly Theives Market) carries all the western gear you could ever want. The sidewalks downtown are still made out of wood, and Route 66 runs through town as Foothill Boulevard. There are horse trails EVERYWHERE.
At the turn of the century (100 years ago, not last year), San Dimas was filled with orange and lemon trees. The town was on the eastern edge of the citrus valley, and there was a large lemon packing plant next to the railroad tracks. The Lemon House, as it was called, was the first in the area that had a box making machine, and was the second to have a moving conveyor belt. Lemons from here were advertised as being in "Frostless Condition," since the weather in Sam Dimas is perfect all year round.
The town has a beautiful canyon, where there is much hiking (and a couple of golf courses), and it used to have a large waterfall at the base of cataract street (before the sheriff's department blew it up). Its still there, but just a lot smaller now.
And yes, Bill and Ted fans, the Circle K is on the corner of Walnut and Arrow Highway...