Ithaca is a great place for outdoor dining.  On the Commons almost all of the restaurants have outdoor tables.  You can't see the lake from here but you can people watch and enjoy some great weather.  Restaurants found just outside the Commons also offer outdoor dining options.  Places like Maxie's Supper Club have covered outdoor dining so even on a drizzly day you can dine outside.

If you're looking for on the water outdoor dining, visit the Boatyard Grill.  In the summer this is a very popular dining choice.  Call-ahead seating is available.  It doesn't always cut out waiting for a table but it can cut down significantly on wait time.  

For the more self-serve diner, pick up a sandwich at Collegetown, Shortstop Deli, or the Smart Monkey Cafe and hit up the local parks.  To name a few that are within a short drive: Stewart Park, Buttermilk Falls, Robert Treman Park, Taughannock Falls, or Ithaca Falls.  The state parks charge a vehicle use fee during the peak season.  The vehicle use fee is per vehicle regardless of the number of occupants in the vehicle and is good for any state park on the date of purchase.  The fee varies for the type of vehicle.  Stewart Park and Ithaca Falls have no vehicle use fee to park or enjoy the facilities available.  Stewart Park offers picnic tables, grills, and public restrooms.  Ithaca Falls does not provide these but it does offer a waterfall view while dining.  Both Stewart Park and Ithaca Falls are carry-in, carry-out parks and do not provide waste receptacles.  

Some of the parks mentioned above also have grills available if you're feeling really self-serve.  For tourists or locals that want to make it an easier night, there are plenty of food stores (Wegmans, Topps, P&C) near these parks where you can pick up supplies.  This is a great choice if you are looking to really enjoy the outdoors.Sunset viewed from Stewart Park