multiple domains for one site

[cerr]

I'm maintaining a website of a lodge which has a golf course, a spa and a restaurant and they all have seperate urls. Now, the main Lodge page contains all the content and for now, the other URLs just forward to the specific pages e.g. spa.com forwards to www.lodge.com/spa. Now I assume that this is not the optimal way to make use of these additional domains... now, how do I best go about this?

I thought about linking from within my lodge.com page to spa.com as soon as people click on the spa link but then again if someone looks for a lodge with spa e.g. it would rank lower as the spa would appear to be seperate from the lodge... how do i best go about this?

Hints, tips and suggestions are appreciated!

Thanks in advance! Ron