Vacation bike rental / routes?

In Kona, rent from Bike Works Kona and ride the Queen K.

x 2 ^^^

I can't think of a bike shop that rents on either Kuai and Maui but you can get some great runs and swims in.

I rented a road bike in Maui a number of years ago, but can't recall the shop.

Here's one possibility:
http://www.westmauicycles.com/

I know you are on the West Side (Lahaina) but I highly recommend South Maui Bicycles in Kihei - Tim, Zach and the guys are great.  They are also sponsors and road support for Sunday's South Maui Triathlon.

You can do a nice loop ride right from their shop, if you want - go up to Piilani Highway and travel as though you are going back to the airport, up Mokulele Highway, then turn left on Kuihelani Highway as though you are going to Lahaina, then left on North Kihei Road, cruising along the ocean and mudflats, back up to Piilani and back to the shop.  Or, cruise around the West Maui Mountains for a more challenging ride, but for God's sake, be careful on the "backside" of the West Maui's - one-to-two lane traffic, winding roads, crazy local drivers and tourists not always watching the road...  But a ride you won't forget - absolutely scenic and stunning.

You'll have to map them to get the distances. 

Congrats on your impending wedding...hope you enjoy your stay!

I was going to mention the same thing.  I've only visited Maui about 4-5 times (I live on O'ahu) but I remember the road to Lahina being extremely dangerous even for drivers if you are not paying attention.  In other areas where there are multiple lanes of same way traffic, or large shoulders, it's great...but PLEASE get some advice from Diane or someone at a bike shop for an exact route you should ride and stick to that.  There are tons of tourists out there that are looking at the mountains or the ocean, and not on the roads like they should be.

Awesome.

Thank you all for the recommendations!

while your in kona check out http://cyclestationhawaii.com/ 
Theyre a great shop, i live on the opposite side of the island and still purchase most of my equipment  from them! 
and yeah, id definetly ride the queen K if i were you. i only live 100mi away from kona and i still get chicken skin whenever i ride there.