In lieu of flying to Amsterdam, I'm planning a more local road trip and am wondering if there are any cities in US and Canada that are significantly better than DC for this type of fun . . .  

Assuming you're refering to carnal fun and not the herbal inebriants that Amsterdam is known for, but Montreal by far. Wonderful city, the adult entertainment capital of Canada.  Many options (use the TER search engine or peruse the MTL board) and with the exchange rate in favor of the US$, donation rates are fairly inexpensive compared elsewhere.  Stay in Old Montreal, pretty central to everything--restaurants, sights, public transit, and the city's massive underground walkway.

Not only this kind of fun- which is more relaxed in terms of regulation, but there is all the other fun that Vegas is popular for.

I've found my hobby dollar to go very far in Atlanta, Dallas, and NOLA.

CAD is $.70 to the USD right now, won't ever be any cheaper in Montreal or Toronto.

Might want to include the Carolinas (i.e., Charlotte, Raleigh) too.