I agree with the others, (though I'd probably struggle with feeling uncomfortable too) but remember as Thorn said - she's not really giving you cash or a monetary gift, she's giving you the gift of being there and sharing this trip, because it will make her happy as well. You will be giving her the pleasure of your being there, so she benefits too.

I am sure she would like nothing more than for you to accept graciously. You can always consider paying her back with a gift of your time spent on a shared activity, rather than in money.