As a long time SB resident and parent of a girl scout here's my suggestions of some different ideas:
To Dos: (1) call Michael Pless at M&M surf school (www.surfingschool.com) and see if he'll donate a surf lesson to the girls or do it for a deep discount! He's a SB local and he'll get all your girls to stand up quickly on his long boards. (2) contact Kim Weston at the Kite Shop on main street (www.upupandawaykites.com) - they organize the kite festival and may have 'loaner' kites or ideas for your troop, (3) The Seal Beach Red Car museum is a quick but interesting site - right next to your GS house - though only open 2 Sat's a month - you could call and see if you could get a special opening for your group (562) 683-1874. (4) ride the Long Beach Transit Passport D bus (only $1.25 per person) from Alamitos Bay (a short walk from the GS house) to either Belmont Shores - then walk around and get ice cream etc.. or all the way to downtown LB and then ride the Aqualink back on the water (prob $5 to ride - and summer schedule not yet posted). (5) go to a classic movie at the Bay theatre - check for listings nearer the time - but they often show fun movies. (6) Call Norm's Big Fish Bait and Tackle on PCH in SB to see if they could do a fishing lesson and provide gear - fishing on the pier does not require a license. (7) Get a couple of cheap boogie boards from Dolphin Market or CVS and just let them goof off on the beach!
As for eating - no shortage of choices - El Burrito Jr is more 'local' than Ruby's (which is just tourists) and probable more affordable for the troop; O'Malley's (while a pub) is a great spot for lunch with a good buzz/atmosphere that our kids have always enjoyed- call Kirk (manager) to see if he can reserve an area/tables for your larger group. Lastly, go to Angelo's Italian deli (on Main st also) and get sandwiches made to go - then go sit in Eisenhower park (at pier entrance) and have a picnic soaking up the local atmosphere.
Enjoy - I love Seal Beach and all it has to share!