Hi That link to "NAGC" is to the US organisation. It's called Potential plus now in the UK. We used to go to lots of fun things that DDs still ask about but it was run by volunteer parents and like a lot if things that has got hard to sustain - no one has the time. The Big Family Weekends were great. I've had more luck being more specific about what my DDs are actually into- take them to lessons or clubs that are in their area of interest. Also develop new interests by encouraging them to try things they don't know about, and find things you would like to do together- nature, music, a new sport. Also watch out for bullying for being bright, all too common in school and can be under the teachers radar.
DD is a member of mensa. She was invited to join last month after taking part in a supervised test as part of a country wide school quiz. I only let her join as I am a member and a family membership is just an extra £20 a year. Not sure that she will get much out of it.