Last week I booked a R/T to Rio using my Skymiles for exactly the dates I wanted in March.
Today I booked the same trip for June. I did have to leave one day earlier than planned and return one day later (poor me) in order to avoid using double the number of miles (SkySaver at 50K instead of SkyChoice at 100K).
Use their online SkyMiles reservation page and you can often fine what you want, except Vegas but with the cheap fares it's better to just pay for the flight and pick up another 4K miles.
I've never had a problem getting SkyMiles flights to San Jose.
Keep in mind if you're flying Delta from ATL to SJO you will have to compete with about 50 other guys for each of 2 daily flights.
I did find SkySaver flights going from ATL to SJO on Feb 21st and Mar 6th.
The best thing about using the Citibank American Airlines Mastercard is their reduced mileage flights that come out quarterly. I've seen flights as low as 22,500 miles from ATL to Cali, Medellin or Bogota for Platinum card holders, 25K for Gold card holders.
http://www.citibank.com/us/cards/cardse ... /globe.jsp