For three of us I budget $100 per day for full days of our trip and $50 per day for three of us for travel days for incidentals. On a week long trip six full days, seven nights for us that is usually $700. Airport parking for example can often run $80 or so for our trip. For me something else that is an incidental is gas and parking charges, but you are onsite with no rental car I'm assuming. But Uber could easily be around the same cost as gas and parking if you use it a bit. We don't buy souvenirs, but invariably someone will forget something and we'll pick up sunscreen, a fingernail clippers, or something like that at Walgreens and that would come out of these funds (You'd think with all the traveling we do this would never happen, but it usually does happen for us). And then there are thing that look fun that we do at the spur of the moment like rent those little mini speed boats at the Polynesian or Contemporary, play mini golf, decide to play tennis on the clay courts at the Grand Floridian, hit a smaller attraction on a non theme park day. I'm not talking all these in the same trip, but these are just examples of incidentals from trips past for my family. My big theme parks are a separate budget item, but I don't budget for smaller attractions and use incidental money for those. If you are doing theme parks every day you don't have a much of this, but we don't do them every day. And if it's a hotel, not a condo we usually leave three to five dollars a day for a tip for the maid. I always tip shuttle drivers a few dollars too.
I find that having a good feel for what to budget for incidentals each time gets pretty easy if when you go on vacation you actually track all your spending. From doing this for my family for several decades, it's super easy for me to budget. If it would be hard for you to go over your budget, I would maybe over rather than underestimate. If it would be no big deal to go over your budget by $400 or so, you can be less conservative and budget what your best guess is with no cushion.