My wife is a Spanish national and I am a US citizen. We live in Spain but are visiting in the States right now. She needs 99 day extension for her visa, and we've been told that if we leave the country and return, they will stamp her passport another 90 days. Does a weekend trip to the Bahamas count as a legitimate departure from the US?
For en ESTA visa waiver, the Bahamas does not qualify - its an 'adjacent island' in the VWP rules and therefore treated the same as a visit to Canada or Mexico. It does not stop or reset the VWP clock.
The closest country which qualifies as a potential 'reset location' is Costa Rica.
A few years ago I was researching much the same thing, and although I can't remember exactly where, I found that the countries between Mexico & Costa Rica don't count for a VWP reset either (or maybe we just didn't want to go to any of them ...).
However, there is no guarantee of a "reset" under the VWP - it's almost entirely up to the immigration officer as to whether or not you'd get another 90 days.
Anecdotally I can say that by alternating between weekend visits to Costa Rica and 1-2 week trips to the UK, a VWP reset for the full 90 days was given by the IO 4 times over the period of about a year (and no uncomfortable questions were asked at a subsequent interview for an H-class visa) - but this might be considered to be going against the spirit of the VWP system, so ymmv ...