You are a lovely person.

A possibility would be to make a schedule with her, according to what suits you. So for example if you are okay with taking her to the thrift store once every two weeks, or once a week, or once a month, whatever it is, you could arrange that with her in advance. Then she has something to look forward to and you have a boundary: The schedule is what you are able to do and it will I expect be easier to say no at other times, if she were to ask.