"Please keep your dog off my grass :-)"

Plus (and this is the important part) a temporary electric fence. One
wire, about a foot off the ground, electrified with something like
a Red Snap'r #66: http://www.stockyardsupply.com/page8/

You won't need it electrified for very long. If it's not a very big
lawn, you should be able to use wooden stakes for the posts, but you
will need to use plastic or porcelain insulators like a real electric fence.

Best regards,
Bob