Mojo Zen ~
Hey there, I’m in Oceanside so am right around the corner.
Best weekend riding? I like to head inland…there is some great riding into Bonsall, Fallbrook, rainbow and points beyond depending on how far you want to go. Here’s a ride I did recently to give you a flavor. http://www.mapmyrun.com/ride/united-states/ca/oceanside/395475878 . Of course, there is the coast, but you’ll find tons of traffic (both bike and car traffic) in the summertime. Also, heading north into Camp Pendleton is a great ride, just remember your ID.
Which bike should I bring (that will be most useful) for local riding: Road bike (Orca) or tribike (P3C) ? Truthfully, either is fine, IMHO. While there is some hilly stuff inland, no really long sustained climbs, unless you want to tackle Palomar. BTW, Here’s a post I wrote on Palomar (http://jpsjourney.wordpress.com/2007/04/23/palomar-mountain/)
But, since I only have one bike (a tri bike), I may not be the best person to ask. 
Best bike shop? Nytro (www.nytro.com) and B&L (www.blbikes.com) are both great.
Best place to swim: Open Water, local gym and city pool? Check out www.carlsbadmasters.com, they work out at Carlsbad HS Mon - Sat AM’s and Mon-Fri at lunch. A very solid group, lots of strong swimmers. Open water, I usually go off of Pine Street & PCH (just south of Carlsbad Village Drive). There is a little grassy area there. Walk down the hill, swim straight out and then north parallel to the beach.
Best place to run…along the beach I’m sure, but where? Anywhere is good. I like to start at Pine Street & PCH and head south along PCH. If you want some trails, check out Calaveras Hills, not a huge area, but enough for a 5 mile run or so. http://www.mapmyrun.com/run/united-states/ca/carlsbad/9369518
I’m in Baja for the 70.3 this weekend, but PM me afterwards if you’d like to get together for some rides or runs or just want some more ideas.