Can I also just encourage you, whatever you decide on, to avoid a surprise party?
When my ILs were due to celebrate their 50th, MIL requested a small celebration (her, FIL, their three sons, and the one spouse -- me). Younger BIL was insistent that we had to do more than that, because other families of our acquaintance had done bigger shindigs, so he planned a surprise party in a reserved room at a local Italian restaurant and invited all of the ILs' siblings and their families. FIL's brother had eight children, for example, so with all of them, spouses, and children, it turned out to be a quite a crowd, probably 100 people.
Well, MIL was quite annoyed. She thought she was just going out for the dinner she'd requested and wasn't as dressed up as the guests, some of whom were in sparkly cocktail attire while MIL was in slacks and a sweater. She sulked, pouted, refused to let pictures be taken.... and then proceeded to over-imbibe because she was so peeved.
FIL, on the other hand, felt the need to make a big, important speech when he saw the crowd. The problem was that he tends to ramble and digress -- a lot -- if he speaks off the cuff. So, the speech turned into a lengthy disquisition on his military service (he was in the Navy when he met MIL) that didn't include a single mention of MIL, the woman to whom he was married and his fellow guest of honor.
This did not improve MIL's mood one bit, and she proceeded to imbibe some more and started crying. YBIL ended up having to drive two quarreling parents home while the rest of us had dessert.
It wasn't the warm family gathering YBIL intended.