It depends on your interests and point of view. I've been to Venice many times and never been bored. And I've never even gone to a fancy restaurant or taken a gondola ride, except for the little traghetti gondola ferries across the Grand Canal. However, art and architecture are among my main interests, I love just endlessly poking around, and I've never gone in summer. In my opinion (and probably the opinion of many Venice fans), Venice is most rewarding when viewed as an experience, not just the must-sees.

If you do get bored and have any interest in art or architecture, you could consider the Accademia Gallery (Venetian painting from medieval through 18th century), the Guggenheim Collection (modern art), the Frari church (wonderful Renaissance paintings), Ca' Rezzonico (18th-century art in a grand palazzo with ornate carved furnishings), or the church of Santi Giovanni e Paolo (sometimes called the Zanipolo; lots of sculpted tombs of doges), for starters. I could go on and on about churches but won't.

Some non-culture-heavy ideas include riding the vaporetto up the Grand Canal and seeing the variety of palazzo facades, or taking the elevator to the top of the bell tower of San Giorgio Maggiore (one stop across from San Marco by vaporetto) for an astounding view of the city. For history, there's the Correr Museum across Piazza San Marco from the Basilica and even included in the same ticket with the Doge's Palace. Or just walk, as has already been suggested.

Or all this just may not be to your liking. Tastes differ, of course.