Hike around, have a picnic and camp overnight if possible. Your body needs time to acclimatize - sleeping, walking around, even shopping at altitude will improve your ability to deal with conditions even higher up. If you've had previous symptoms of altitude sickness, you might wish to contact your physician before your next trip. At altitude, a good rule of thumb is to sleep no more than 1,000 feet higher each night above 10,000 feet. Your best bet is to train high and sleep low, meaning set up a low base camp, take a hike to elevation, and return to base camp to sleep.
Dehydration can increase the risk of developing altitude illness.
During all hiking, you should be monitoring your water intake by your output. If you are not urinating regularly, or your urine is dark yellow, you need to drink more liquid. Plan your goals keeping altitude in mind. Although you may be able to flash 5.11 at your home crag, you might have to work harder on a similar route that is at 12,000 or 13,000 feet. Altitude Illness, Prevention and Treatment by Stephen Bezruchka, 1994.