If you are driving from Girdwood to Seward on the 15th I would not plan the sea kayaking trip that day. Do the sea kayaking to Aialak Glacier on Monday. It is a pretty drive with lots of stops between Girdwood and Seward - Wildlife Conservation Center, Begich Boggs Visitor Center, Byron Glacier Trail, perhaps a stop in Hope, maybe other scenic stops? We left Girdwood mid to late morning and didn't get to Seward until 6-7pm. We did make quite a few stops and hiked part of the Winner Creek Trail (too much snow to get very far though).
You can stay in Girdwood or in Anchorage the night of the 18th. Not really much time to see Anchorage on Thursday morning. You should probably be at the airport by 10.30am to return your car rental, check in, go through security, etc. ANC airport can be quite crowded in the summer with all the tourists, including thousands of cruise ship passengers.