My sister celebrated her bat mitzvah last summer ... her service was a bit shorter, only an hour and a half (and she only lead about 30 minutes of it, but it's still an accomplishment), and her party was something both the adults and kids could enjoy. The party was held at a local hands-on museum that will let it's members rent the entire facility for private parties. All of the regular museum exhibits were available ... the adults had a room downstairs with lots of tables and a fairly fancy food service (about what you'd find at a wedding reception), and a small dance floor, and jazz trio. For the kids, there was a room upstairs with pizza, pop, and a dj. We also had a karaoke room available ... one of the little home karaoke machines hooked up to a tv so you could see the lyrics, and our vhs camcorder to let the kids have a tape of their "performance". That went over fairly well among the kids. A lot of people said the museum was a great/unique idea for a party.
As far as presents, when I celebrated my bar mitzvah, the average gift was in the $50-75 range ... some people gave it as a check, others gave savings bonds. I got some gift certificates to local stores, 1 or 2 nice pens, and even some clothes. $150 would have been a pretty high dollar-value gift, certainly higher than average