Soft and gentle:
* Thump-thump
* Tap-tap
* Pat-pat
* Slap-slap
Medium intensity:
* Whap-whap
* Bop-bop
* Thud-thud
Energetic and excited:
* Whizz-whizz
* Swoosh-swoosh
* Bam-bam
* Boom-boom
You could also combine these sounds, like "thump-thump-swoosh" or "whap-whap-bam." Ultimately, the best sound will depend on the size and energy of the dog and the intensity of the wag!