If you have a little bit of flexibility with that 6pm time, you could head to Whittier and do one of the glacier cruises.
There are several options, some that are shorter or get out earlier than others. (Some may actually let you get back to ANC before 6pm.)
Plan your return time around the
one-way tunnel schedule between Bear Valley (Portage Glacier area) and Whittier. The tunnel is about 45 minutes south of ANC, and Whittier is just on the other side of the tunnel.
You could also just
drive. Head south down to Girdwood, Portage Glacier, and maybe turn around at Turnagain Pass--or keep going to Seward if you have the time (about 2.5 hours south of ANC without stops). If you make it to Seward, check out Exit Glacier. Or head north, perhaps out to Matanuska Glacier or Hatcher's Pass, or maybe to Talkeetna for some spectacular views of Denali if the weather is clear.
Although it's a bit of a waste if you have a car at your disposal, you could stay in ANC and check out the Alaska Native Heritage Center (definitely worth a few hours--I could sit and listen to the storytellers talk about life in their Native villages all day). Poke around downtown a bit (always stuff happening on the streets near 4th and E, and there's also the museum). Consider hiking up Flattop--an easy 20-minute drive from downtown and a relatively easy 1.5-hour round-trip hike with some stunning views.
Here's a fairly good list of other options to do. Anything there strike your fancy? Let us know and maybe we can help narrow it down.