ESPN only takes reservations for lunch, Mondays through Fridays. No dinner or weekend reservations. IMHO, unless there is a major sporting event happening, you don't need a reservation. DS9 and I walked in twice for lunch and dinner during our last trip the week before Christmas and had no trouble being seated. They also have a bar (and the kids can sit there!) if you just want milkshakes. And you can certainly just step inside and buy a t-shirt (I'm almost positive they have them. I know they have baseball hats and tervis cups because we bought some!). I, too, would not waste a TS credit there. When DS9 and I had lunch there, I had a salad with chicken and he had the kids burger (which came with a cupcake!) and our bill was LESS than our BOG lunch. Walk up, pay OOP and enjoy!
Also, I would really recommend going to the Boardwalk in the evening as that's when all of the entertainment happens, they sometimes show movies on the Village Green and the carnival games will be open (bring cash - they don't take MB's!). And if you really want milkshakes, get them at Ample Hills and not ESPN Club. Ample Hills has arguably the best ice cream on property. Again, if the menu at ESPN doesn't appeal, I'm sure they would have no problem with you looking around and buying a souvenir or just sitting at the bar even for a soda, beer and/or fries.