Every department is different. Everything I'll write is specific to my (top 5 CS, public, RU/VH) department. And while I know quite a bit about the process of hiring new faculty, I do not know the precise details of any recent startup packages (except for salaries, which are public). My department head manages all startup negotiations directly.
Our formal startup package is a bag of cash (formally: "unrestricted funds"). New assistant professors can spend that money to pay their own summer salary, support students, but equipment, pay for travel, and so on, in the hopefully brief period before they have active research grants. Precisely how and when they spend the money is up to them. Moreover, this money does not "expire"; I still have a bit of money in my 15-year-old startup account. Again, I don't have an exact figure, but I'd guess somewhere in the low six figures.
Our teaching loads are already low — 1 course per semester — but there are courses, and there are courses. The de facto standard is for our assistant profs to teach mostly smaller graduate courses (in part to attract students), with at least one semester without teaching in the first two years (to work on research and grant proposals). However, it is important for assistant profs to teach a few required undergrad classes before tenure. We usually pair new instructors with more experienced faculty the first time they teach such a class. I don't know how much of this is formally described in our offer letters.
The other department that offered me a job n years ago had much more restricted startup package. They asked me to give them an itemized budget listing specific equipment (which they would buy for me), a specific number of student semesters, specific dollars for travel, and so on. At the time, I found this approach unreasonably inflexible, but it's much more understandable now because...
At least at my university, startup packages in each department are funded by indirect cost recovery from grants originating in that department — a small fraction of every grant I obtain goes to CS startup packages. Thus, departments with more grant activity can offer larger and more flexible startup packages. This is orthogonal to faculty salaries, which (at least at my university) are paid entirely out of state money, making them much less flexible.
Second, what types of changes are reasonable to negotiate? Can I ask for more summer salary support while still conveying to them my desire to obtain grant funding as quickly as possible? The offer already includes a reduced teaching load for the first few years --- is it also reasonable to ask for an extra semester off prior to a tenure review?
It's perfectly reasonable to ask for anything, as long as you understand that the answer may be no. Do not be afraid to ask stupid questions. Everybody knows that you're new and don't understand the department's budgeting constraints and standards. That assumption of naïveté gives you the freedom to ask for things you think are "obviously" unreasonable. They probably are unreasonable, but they might not be.
Also, remember that if you're already negotiating startup packages, the department has already decided that they want to hire you. That decision requires developing consensus among the faculty, getting approval from upper administration (not just for hiring you but possibly for hiring anyone), and filling out a metric buttload of HR forms. (Warning: Some universities still use imperial buttloads.) There is almost nothing you can ask that will change their minds about the basic offer.
Don't worry about giving the impression that you're not interested in applying for grants. Any such impression will vanish the moment you actually start applying for grants! (Which is soon, right?)
Finally, the best way to find out what's reasonable is to ask the other assistant professors in your target department about their startup packages. Just straight-up ask them. They won't all answer, but some will.
