Here is a site that delivers rental bikes anywhere in Portugal
and Spain. They have a Lisbon office. Deliver to your hotel etc.
at an extra charge.

Hourly, daily and the weekly rates. The rates don't look cheap
to me, see...

http://www.bikeiberia.com/rentals_rates.html

Around 75 Euros a week for a kids bike, 100 for a standard bike,
more for mountain bikes.

Pre-registering is required at

http://www.bikeiberia.com/rentals_anywhere.html

How much do bikes cost in the supermarkets now? Saw some for
sale in Le Clerc's just the other day.