I think the pants & jacket combo would look spiff, & be a great way to showcase the New You. But if you don't have them, don't worry about it. It's far more important to be comfortable & able to enjoy the occasion, than being all self-conscious.

Why not go with your summer dress idea? You could be cool & comfortable, yet still show that you'd made an effort. Also, dresses tend to be a bit more forgiving of size changes, in either direction.

The main thing is, have fun with your dad! [Is he tall enough to provide a convenient corner for you to lurk in? ;)]