The museum's architecture isn't the most updated. Compared to the Cleveland Museum of Arts, the NH Museum feels dark and enclosed....which is why I like about it - it's perfect to put your tired toddler to nap.
I let her run around the museum, grab a quick bite at the cafe, put her in the stroller and walk about the tunnel leading to the planetarium. It is dark with eerie (or soothing, depending on your taste) music. Perfect for a quick power nap.
If you have the membership, the planetarium show and parking is free. You can also bring in guests for free. My 15 months old watches the planetarium show without fuss - she didn't fall asleep, nor did she disturb other patrons. The show was 40 minutes long.