What to do with an old iPhone without SIM

I have an iPhone 3GS I no longer use as the battery does not hold and I have a new iPhone 4. I was thinking of using it from time to time at home just to browse or read emails without cellular network (i.e., with no SIM).

After a long time I switched it on initialized it and realized that I need a SIM to activate it (although I already activated it years ago).

What are my options? Can I activate the old 3GS with my current SIM or will I deactivate my new phone? Should I try to get a Phonebook SIM? I would prefer not to jailbreak it if there is a way.

Edit: the phone was officially unlocked after 2 years of contract.

If the phone is unlocked, you can use it with any SIM, even from a different GSM operator or even a SIM that is not in service (obviously in this case it won't work as a phone). If you don't have one or can't find one, you may be able to get prepaid SIM for cheap from an GSM operator.