Not sure on favors...most parents don't appreciate the junky little doo-dad toys. But for the Thank you cards..take a good group photo or a fun one of each party goer with the birthday child..especialy if you do something fun with bowling shirts/tshirts for them to keep. Then do each picture up and put into the thank you card! Each party-goer will get to keepsake picture from the fun day at the bowling alley! Some places offer a certificate for the kids to come back for a free game (they want the business of course!)...if they didn't get that at the party, slip that into the Thank you card as well.
For my daughter (who is a teen now) we always did a fun useable keepsake thing. Her 4th birthday we had a pony ride/face painitng/small petting zoo place come to the house for their stuff. My mom and I made super inexpensive and simplistic cowboy/girl vests, bought straw cowboy hats and red bandanas and sheriff badges at the local party supply store. Everyone went home as a cow-poke that day! A few candy treats tucked into their hat or bandana and they were set! Everyone got a thank-you card picture of the group with their costumes and also individual ones on the pony ride! It was a hit! Get creative!!