There's not really that much more, the National Space Centre is fantastic, and The Curve theatre is rapidly gettin a good reputation amongst producers and comoanies. Apart from that you've not missed anything.
There's the enormous covered market, selling exotic veg you've never heard of before.
New Walk Museum, with the dinosaur and the Mummies.
The Newarke Houses Museum, with the clothes of Daniel Lambert, local famous Very Fat Man*.
The Jewry Wall Museum, with the foundations of the Roman baths and the largest standing portion of non-military Roman Wall in the UK.
The Great Central Railway.
Bruccianis Ice Cream/coffee shops.
The Pumping Station, with the 'Journey of a Poo' display.
Frankly, I'd give the Gas Museum a miss, I always found it a bit dull, and the smell of gas made me feel a bit sick. Although they do have a gas hairdryer...
*He had something glandular, and was hugely obese. He was a national celebrity in the early 19th C, and featured in cartoons and so on.