I find open RDS's work nicely with the p90 since it has a CQB feel to it, though they have more parallax to deal with. For looks alone, just about any scope will look good on the p90 though, even the one's longer than itself!!
I personally use the ATN ultra sight (
got it from this thread), its about $90 and works well. If you want better, you can get a Mojji one for about $120, or if you want cheaper, you can get a generic square RDS for about $50.
If you win the lottery any time soon, you might even want to go with a EOtech holosight!