I am using my Cappy without phone service, mostly as a music player and navigation device. When I am not using the GPS or wifi I keep it in airplane mode. I have had 50% battery remaining after four hours as GPS, music player for an hour, 30 minutes on G+, some wifi use to update apps; probably 1.5 to two hours screen time in total. I do keep bluetooth turned off unless in use. Likewise, since I don't have a sim card in it, I keep data off with 3G Watchdog - probably not necessary but a holdover from use on a low data plan.
On other days I have had over 3 hours of screen time using the GPS with over half of the battery remaining. This is the original battery in the phone when purchased in Nov 2010. I do use TiBu to keep some apps frozen. This is on stock, rooted, 2.3.5. (see sig) All of this leads me to believe Captivate Touch would be great for your purpose, as would most any ROM.
The reason I haven't installed Captivate Touch is that I want the option of using this as a back up phone for myself if necessary. I also have the option of trying out Straight Talk on it for myself or as a loaner to family
TW 4.1.2, stock, rooted,
Samsung Captivate SGH-i897
Currently CyanogenMod 10.1.3
resurrected from softbrick using 4-2ndtwin's Heimdall OneClick to stock Gingerbread - 2.3.5 - kk4
ROM history Eclair > Froyo > Gingerbread > CM9