Just the city center is worth visiting! Its a good town to stop by for one or two nights, but not more!
For a taste of local food try Pancho's Restaurant and Bar just one block away from the main square. We enjoyed some really good food there!
Very recommended, if you are not into boring Cancun or shallow fun in the bars and discos in Zona Hotelera; definitely worth to see the nice architecture in zocalo and to eat delicious food.
..; Chichen Ytza is only 80km away from the city (40-45 min by car); car rental quite cheap - 40 usd/day
Merida is a small town, and it can be a little bit boring. There are some good restaurants, and they are not expensive but serve really really nice food.
Going to Uxmal may be the best course of travel.