My suggestions:
email opt-in - I don't recall seeing an opt-in! You could write up a nice pdf such as "Top 5 Ways to Save a Bundle on Your Next Vacation" or something and offer it as an inventive to opt-in.
With all that traffic you could probably build a nice list, and then just give them some lovely content in the in-box with the occasional offer. Setting up some worthwhile emails shouldn't be difficult for you. Follow up with very brief surveys so you can get detailed information on people that opted in and then send campaigns their way accordingly.
Page speed optimization and cdn: get w3 Total Cache plugin, it should improve loading no matter what. And is CDN-friendly in the event you want to then serve certain media using a cloud service. Guess I'd recommend you consider using a cloud service to host your images (and any videos...). You can get a plugin that will automatically host additions to your Wordpress media library on your cloud service.
Amazon seems to be going down a lot, I suggest you look into Rackspace's Cloud CDN. Also consider Cloudflare and Encapsula, these may help site loading times and will definitely improve security. Either of those can be used with or without serving your media on Cloud. I intend to try a Rackspace Cloud + Cloudflare setup on a new site soon.
Videos: Add quality videos (especially quality videos you host, use Rackspace CDN or Amazon ). These would be great for your visitors and should provide rockin boosts to SERP rank.
