Glad the neutering went well.
Surprisingly mental games can do as much to tire a doggie out as physical activity.
Try teaching your guy to do "math". One finger raised=1 bark. Once they have that down pat...go to 2 fingers=2 barks, etc. It can be a real show stopper to ask your sheltie what 2+2 equals, show him 4 raised fingers in a way that only he can see, and have him bark the answer 4. Kids in our previous neighborhood believed that our Rory (past sable sheltie boy) could do addition, subtraction, division, etc...and even when I showed them how we were doing the "math" they refused to accept that Rory wasn't actually coming up with the answers on his own. He was their hero.
Dogs can distinguish sizes when trained with items that are the same except for size. I have 3 antique goat bells that my Grandmother brought to this country when she came from Spain. I built a frame that stands on the floor and has 3 hooks for hanging the bells...one of the bells is very small, one is a bit larger and one is large. Every sheltie I have worked with on these bells can be directed to ring any of the sizes on command by pushing the right size bell with her/his nose, or touching with their paw. To start I would put a bit of PB on one size bell and work with that size until they learned the "name" for that size...ie either "small bell", "medium bell" or "large bell". Then add a second size to work on, etc. It doesn't need to be bells, anything that you have in different sizes works fine and you can direct your guy to show you or bring you the size you want in that group of items.
Dogs also fairly easily learn names of different toys or other items...and you can institute a "trade game" where you ask your dog to bring you a certain toy/item and tell him you will give him a cookie in trade.
There are literally zillions of games/tricks dogs can learn by your just making up things to teach them using household items, your hands, etc. And it is so much fun to see them "get it". However, there is one trick I definitely would NOT recommend teaching a sheltie...and that is how to open the frig!!!!! LOL
Trini