I'd agree with all the above.
My sister has done this twice now and in a very intelligent way - her attitude is simple - she is their dad's friend and is totally available to be his kids' friend too. She just acts like she likes them, enjoys being with them, is interested in their lives etc. She knows she is not on the same footing as a mom, and doesn't pretend to be.
Ask them lots of questions about what they do and what they like, and try and remember the answers for next time - they'll be so impressed.
And rather than buying gifts, my sister got herself clued up on what kids are into by looking on Amazon at the bestsellers (books, toys etc) for their age group. They thought she was so cool to know all that stuff! She even got herself a DS so she could play their games with them. Maybe that was going a bit far....
But they love her.