Can't give you any specific suggestions, but wanted to make sure you got at least some general help. Hopefully a local or someone nearby will chime in later with some more specific suggestions.
Here are some ideas and/or web resources you might want to check out (in no particular order):
1. Obviously there are some things on the Purdue campus that they can check out (beyond the "usual suspects" of a campus visit). I'm presuming they've checked out the Purdue webpage, but just in case they haven't, here's a link to the "visiting" page which includes a link to an on-line visitor's guide:
2. Prophetstown State Park and the Tippecanoe Battlefield are both nearby.
3. Here's a link to the Tippecanoe County "visitor's" page which has some links to other resources:
4. Here's a link to the actual Tippecanoe County Convention & Visitor's Bureau page:
5. Lastly, for eating ideas, here's a link to the Urbanspoon page for the Lafayette area:
Urbanspoon is my "go to" restaurant site and most of Indiana (and it appears the Lafayette area) has the critical mass to make this sort of crowd-sourced website worthwhile.
I think between the information on the above pages and/or pages they link to you'll have a pretty good list of things to see/do.
Beyond that . . . there's not really much between Indy and Lafayette/West Lafayette - at least not on the direct route (I-65) - it's only about 60 miles and there is really only 1 small town on the way (Lebanon). Thus, in terms of things "on the way" I'd suggest either things in Indianapolis or taking a non-direct route (even then there really isn't that much on the way).
Don't have time to go into all the things Indy offers right now, you might check out the Indy forum for ideas. I'll try to post a few other ideas later. On that note do they have any particular likes/dislikes? This will allow me (and others) to tailor suggestions that are more likely to interest them.
Hope this helps,