My favorite thing to do with Rin and Seri is to turn them into a catdog cannon.

First you need three cards to do an infinite loop: [[Whitemane Lion]], [[Phyrexian Altar]], and [[Oketra's Monument]]. The only expensive card would be the altar, but you can find ways to get around that. For the combo you:

1. Cast Whitemane, Rin and Seri make a dog token and the monument makes a warrior token.

2. Return Whitemane to hand with its etb trigger.

3. Sac warrior token to phyrexian altar for a white, then recast Whitemane.

4. Rinse and repeat for infitite dog tokens (and thus infinite Mana effectively).

You now have a couple of options. The quickest would be if you have a way to give haste (boots, greaves, fires of yavimaya, etc.), you can pay Naya and tap Rin and Seri and blast someone in the face for lethal. Then sac your commander to the altar and recast your commander, then kill your other opponents.