fun, inexpensive restaurant with some seafood?
It's hard to be specific without some idea of taste and budget. Inexpensive is such a relative term and fun, which may be far more subjective, always means distraction to me. If a restaurant has good food, it's a fun place for me, if it has medicore food, I feel I've wasted a meal. When I see the word "fun," I"m led to suspect food is sceondary and therefor my recommendations may not be what is sought.
I recommended a Chinese restaurant on Laguardia Place a bit north of Houston. She-She China. It's very inexpesive, has excellent food, although red meat is not the strong point of Chinese restaurants, and above all else has been rather less than full, so it's both easy to get a table and a good place in which to hold a conversation.
Far less quiet, more expensive, but still one of the better values in NYC is the bar room at Gramercy Tavern. This is an upbeat room and should qualify as fun. You can expect to wait at the bar for as much as an hour at peak times, but as I say, the food is very good and the prices are very reasonable.
Also not at all inexpensive, but an incredible value for the quality of food and cooking is the new Blue Hill on Washington Place. This, even more than the bar room at Gramercy Tavern is a destination restaurant and a place to focus on the food as well as the conversation. It's a place where anyone really interested in food, should find the conversation turning to the food, no matter what was on your mind when you entered the restaurant.
When making recommendations it's far easier for me to speak about fine food than either fun or inexpensive. Cello, uptown has a great value $35 prix-fixe lunch. This is not a casul restaurant. It's quite elegant and the wine list is very pricey, so that sort of balances the cost of the food. There are few wines below $50-60. This not to say they're overpriced, just that there are few inexpensive selections. It's also primarily a fish restaurant and they only had fish and one chicken dish on the lunch menu. Dinner at twice the price will likely have lamb, beef and/or veal.
The Brasserie in the Seagrams building comes to mind as relatively inexpensive. With it's decor it may well qualify as fun. The French fries have been incredible. Mussels in a spicey red sauce with fries at about $14 was one of the best buys in NY. The mussels were very fresh and perfectly cooked both times I've had them. Of course that is not what either of you are looking for, but they do have both red meat and fish on the menu.