Interplanetary travel is a staple of sci-fi, whether you need generations of people to go from one end of the solar system to the other or you can do it in the blink of an eye, it's kind of unavoidable if you want to tap the "final frontier" angle or involve any aliens, so how do we go about establishing a consistent and reasonable means of interplanetary travel?
The first big question is what kind of FTL do you use? Every setting is very different on FTL, Elite Dangerous's Frame Shift Drive allows relatively slow but still FTL speeds, reaching 2, 3, or even 4 times the speed of light when free of any immediate gravity well, Starsector allows ships to reach appreciable fractions of light speed under their own power, usually inhibited by local space hazards and how much inertia the ship builds up at speed, while FTL travel requires static "warp points" found at select places around each star system, so travel between planets, although accomplished in mere minutes real time, requires literal in-game days. Some settings don't have FTL at all, with ships simply being built around the unfortunate fact of cryostasis being mandatory for any sort of travel through deep space. Like a lot of things what you choose as best depends on your storytelling needs, being able to FTL between planets is pretty handy if you want to jump between them frequently, it keeps stories from seeming locked inside of a starship, but if you want them to seem that way "slowboating" may be your choice, while no FTL at all is arguably best for fully exploiting the story angle of being (relatively) alone on a hostile alien world.
Interplanetary FTL is very straightforward, there are usually no explicit drawbacks because FTL drives almost always already account for negating collisions with space debris, however it may be prudent to write-in that jumps require a certain distance from the planet to be initiated, or that the power demand is such there is a noticeable charge-up and/or it requires non-essential systems like shields and weapons to shut-down for a brief window.
Slowboating with limited FTL is a pretty realistic approach without doing away with FTL entirely and having to deal with how dramatically that colors a space opera. Depending on how you want to run things travel between planets can take minutes (Frame Shift travel in ED is distinctly different from warping), hours, or even days (many a delivery in starsector has been soiled by how long it takes a superfreighter to drag itself from a warp point to the destination). I would consider myself a little biased towards this, I consider it pretty reasonable for a craft to need a few hours to travel from one planetary orbit to another, even when the two planets are relatively close within their respective orbits, though there is the concern of the faster you go, the more damage any amount of debris can inflict when you hit it, at relativistic speeds a speck of iron dust can slam into a starfighter with all the force of a railgun slug, this obviously kills the fighter.
With no FTL whatsoever you will most likely be looking at days if not months to traverse the distance between planets, partly because ships will be optimized for redundancy, durability, and longevity over actual speed, and partly because you have probably committed fully to the school of hardcore sci-fi with no shields, applied phlebtonium or anything of the sort, which amplifies the threat presented by colliding with pretty much anything at a high speed. Seriously at a high enough velocity anything can cause fatal damage, even to a massive starship.
I just wanted to lay out these thoughts somewhere concrete.
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