bike rentals in umbria

How about telling us where exactly in Umbria first and what exactly you want to know about?
Your question is like asking "who can tell me about daily bike rental in the Bay Area".
Then when you know more detail we'll let you know.

And since tomorrow I'll be working in Santa Cruz all day can you tell me about the weather in Santa Cruz? Was it nicely cool and foggy today or just as hot as across this side of the hill?

As Roberto replied, need a bit more info, BUT, if you are in Pienza, you might check out
http://www.cicloposse.com/day-bike-trips.php

Not sure if you'd enjoy a guided trip to see a bit more in the day, or just want to get on a bike and explore on your own.

We've used Cicloposse 2 times on self guided 10 day bike trips, once in Umbria, and the other time in Maremma.

Really nice people. You can email Giuiliana and see if they will rent a bike to you for a day without a tour, if that is what you want, or she may have some suggestions for local bike shops in the Umbria region that rent bikes.