At HAN, stay airside—compact terminal with dining is safer than risking traffic.
The round-trip commute from HAN to the Old Quarter takes 90 to 120 minutes, leaving almost no usable city time for a 3 to 6 hour layover after clearing immigration. Stay airside at the airport. The international terminal has cafes and a lounge area. Use the time to eat, rest, or visit the lounge if you have access.
Take Grab from HAN to Hoan Kiem Lake (45 to 55 min, VND 280,000 / $11). Walk the lake circuit (3 km, 40 minutes), cross to Ngoc Son Temple (VND 30,000, 5 minutes by bridge), and explore the Old Quarter grid of narrow streets. Pho Gia Truyen on Bat Dan Street is one of the best-known pho shops, open from 6 AM to noon with bowls for VND 60,000 ($2.40). Allow 90 minutes for the return and a 2-hour airport buffer.
The Temple of Literature (Van Mieu, VND 30,000) is a well-preserved Confucian temple and Vietnam's first university. It is 20 minutes from Hoan Kiem by Grab (VND 60,000). Ho Tay (West Lake) is 10 minutes further north and has a pleasant lakeside promenade. Combining Old Quarter, Temple of Literature, and West Lake fills a solid 5 to 6 hour city day.