The mustang coupe has a decent amount of trunk space. Enough for 2 full sized suitcases, and some stuff on top. The back seats also fold down if needed. Probably better to use the seats there for storage, as noone can really fit into them comfortably.

The convertibles do sacrifice a lot of trunk space, so I'll second the bad idea for that option. You might get a suitcase in the back, then another in the rear seats.