Weeza13, honestly if I were you I would smile and say thank you and then just sort through the clothes for anything you actually like and bag up the rest and store them in a wardrobe or cupboard.
Firstly, no one is going to see your little one every single day except you, so they can't possibly know what he wears every day. In my experience, people are just happy to feel that they are helping and contributing. We were given absolutely shedloads of secondhand clothes and equipment, and no one ever asked if it was being used.
Secondly, you might well find that you can use some of the initially unwanted clothes - it's always good to have backups with a newborn. We had a load of vests that had gone through several children, and I am sure I remember that I threw one away rather than washing it, after a particularly explosive newborn nappy drama. In the early days you may occasionally get through several in a day.
Dreamphone, it's great that you've got all those things! I never had to think about choosing a pram either - just got given one. You'll probably meet loads of new parents once you've had your baby and be able to examine all their prams anyway - if you decide to just have the sling initially. I remember noticing prams SO much more than I ever had in my life before when I was pregnant last time!
I use a stretchy Kari Me sling around the house - picked it up secondhand on eBay after recommendations on Mumsnet. It's been brilliant, and one great thing is that you don't have to keep undoing it and re-tying it to get the baby in and out. A friend of mine had a stretch sling that had to be unwound and untied each time, which was a bit of a bugger.
Wow, generous grandparents, mssleepyhead! No experience of those but what a lovely present. 