I think dinner is a great idea, but if they live far away, that's a problem. If there is something tangible and appropriate that you used the gift for, invite them to see it or send a photo. This may not be possible ("Uncle Rich, we'd like to invite you over to see our paid-off credit card bill.") But if you have bought a house or a car or a beautiful pice of art, you could invite them for that. "Uncle Rich, your generous gift let us get rid of that unsafe clunker and buy a decent car. We'd like to invite you and Aunt Charity to a picnic in Beautiful Mountain Park next weekend--we're driving, of course!" or "We'd like you to be our first guests in our new home." or "Dear Uncle Rich, your extremely generous gift enabled us to buy this very special painting by Toulouse Da Vinci. Here's a picture of it hanging over our sofa and we think of you each time we see it." or "Dear Uncle Rich. Thanks to your generous gift, we are taking the honeymoon we never had. This postcard shows the lovely beach. We just had to let you know again how much this means to us."
If you invested the money and you think the relatives would get a kick out of it, you could send them a cute piggy bank.
I said "appropriate" because it's important that the givers don't think you wasted their generous gift. If they know you have college loans to pay off, I wouldn't send that postcard from the Riviera.