You don't do groceries much, but you have 2 young kids? Doesn't make much sense to me, but whatever.
Look, your best option is to buy $500 visa gift cards, be it from a grocery store or elsewhere. Boom. Sign up bonus obtained. You can then use the gift cards for every day use, or you can try to buy money orders with them to cash them out. Your choice. Depends on what you have in your area, and there are tons of threads about all of these options in this forum, many with wiki entries from people who have been doing this for a while and for dollar volume way, way above $3k.