Gwalior's Sun Temple is modern, not ancient, but it is a popular local stop and an easy add-on if you have transport. Built near Morar, it takes inspiration from the far older Konark Sun Temple in Odisha, with a chariot-like form and a bright, orderly temple compound.
It is not in the same league as the fort monuments, so keep it for a second day or for travellers with a particular interest in modern Hindu temple architecture. The site is clean, active and straightforward to visit.