We also stayed at the Westin Maui and it was probably my favorite resort out of the 4 that we stayed at during our trip, including Aulani!
With only 2 nights at Aulani, I'd suggest just enjoying the resort and Ko Olina, which is walkable. Any place worth going is going to be logistically challenging and expensive to do without a car. As mentioned, there's a few restaurants across the street - we loved Monkeypod and they had a great happy hour while we were there. There's several restaurants between the Four Seasons and the Marriott. There's Roy's for fine dining. There's Paradise Cove luau in walking distance. You can't go wrong with any of those.
I know that Aulani is a destination for us Disney fans, but honestly, Waikiki is better for a two night stay and no rental car for two adults IMO. Ko Olina is a dead zone after 8pm in our experience. Waikiki is entirely walkable with tons of restaurants and bars, and some night life. It would be easier to use tour companies or ride shares from Waikiki if you wanted to venture out some place. You could even Uber up to Aulani to check it out if you wanted to, although I believe only Ulu Cafe is open for non-guests at the moment. If you're there when the spa opens, you could also book a spa treatment as a non-guest to check the resort out.