I think you have a lot on your to-do list, but it can be accomplished.

Presumably you want to stroll along the lake somewhere between Oak Street (to the north) and Roosevelt (to the south). Depending on your budget, you might consider drinks or dinner at Spiaggia/Cafe Spiaggia or J. Parker. (J. Parker is actually about 5 minutes north of Oak Street via cab.) Spiaggia is located adjacent to an underpass that will take you to the beach/lakefront and is also at the north end of the Magnificent Mile. (It's not far from Gibson's either.) I'd probably ditch the idea of a boat ride. I *think* most of the evening lake cruises are dinner cruises, so you're probably looking at a couple hours for that.

None of the Blue Line L stops are particularly convenient to the Magnificent Mile. The best you'll do is into the Loop and then walk/cab/bus north or east to the lakefront. Depending on when you're ready to head to the city, you might just want to do a cab. (You can easily check Chicago traffic conditions on Google.)