Lafayette’s one of the few places where you can hike EBMUD land without a permit. And it’s extremely popular: at 10am, the parking lot was already almost full. Most people come to do the paved Lakeside Trail, which is short and flat; the Rim Trail is a lot hiller and a lot less crowded.
Parking here is expensive: $1/hour on meters, with a 2 hour maximum, and $6 all day. And I suspect that two hour maximum is no arbitrary choice: unless you’re really pushing it, it takes a shade over two hours to hike the rim.
I escaped the parking fees by parking on Mount Diablo Boulevard, down towards downtown Lafayette, and walking the 10 minutes to the entrance. Roadside parking is a mixture of 2 hour and 4 hour zones, but is unrestricted on Sundays. A number of hikers in our group parked on residential streets around Village Center, south of Mt. Diablo Boulevard.
As for the trail itself, bahiker.com has it right: it’s a rollercoaster, with never a flat moment. We took it counter-clockwise, which is probably the best way to go: only one really steep climb, a few steep descents after passing the Rheem reservoir, and it finishes with a gradual slope down to the lakeside.
The Rim Trail is relatively short — a bit under 5 miles — but it’s hard work. Not many of the trails are signposted, but for the most part it’s easy to follow: forks left and down will take you down to the lakeside, forks right and up keep you on the rim.
I would do this one again; maybe combining it with a second cool-down loop around the Lakeside Trail.