The Treadmill
The most obvious alternative is the treadmill. If you have one, great...you're all set. If you don't have one but you belong to a fitness club or gym with treadmills...then that is a logical alternative...provided you can get there. Running on a treadmill is not exactly the same as regular running...because the treadmill essentially throws your leg back...but it is pretty close. A 1% grade on the treadmill is roughly equivalent to running on flat ground outside.
If you find yourself having to do a treadmill workout...and you have a challenging workout scheduled...you can make the most of it by using the incline function on the treadmill to effectively do some hill work. This is an especially good opportunity if you live in place where hills are hard to find...like the Cape. I believe treadmill running at incline is more like "real" running.
One of my favorite workouts is to warm up for 20-40mins...with a steadily increasing pace at 1%...starting at about 9:00 per mile and building to around 7:30 pace...give or take. Then I do ~5 x 5mins @7:30 pace @5% grade...with 2-3mins recovery at 1%. After that I cool-down with easy running for 10-20mins. By the time I'm done, it is often 90mins in total...but it doesn't feel like it. The start is tedious...but once I get into the hill repetitions...my mind comes more into the moment and time flies.
You can do something similar. It doesn't have to be anywhere as long or challenging...and you can, of course, make it much more challenging. Also you can do all sorts of different combinations. For example, you might warm up for 15-20mins at an easy pace and then run a 1min "hill" at the top of every 5mins...giving you 4mins on the "flats." Maybe you do that 6 times. You end up doing 6 hills in 30mins.
Here's a chart that can help you determine the effort of running at various paces and grades on a treadmill. For example, running 7:30 pace at 5% grade is equivalent to running 6:32 pace outside.
Other Gym Equipment
What if you don't have a treadmill? Maybe you do have access to an indoor bike, rower, air-dyne, elliptical, stairmaster or other apparatus. These are all good alternatives. If you are using something that you do not normally use, it's probably best to go at a low intensity...and for not too long. I'm really no one to talk. I once bought a Sears exercise bike...brought in home...and proceeded to ride it for 3 hours straight. Not only did make myself a bit sore, I literally wore the bike out. This was 1980 or so...they weren't built so hot then...hee hee.
Indoor Circuit Training
Another fun alternative can be an indoor circuit workout. Again...a bit of caution is advised. Pick movements that are well within your capability and strength. If you never ever do upper body stuff like push-ups or pull-ups, you might want to exclude them from your circuit...or include them in very small quantity.
A 20 minute beginners circuit workout...for a runner...might look something like this:
0:00 - 1:00 Jog in place
1:00 - 1:30 Hop in place or jump rope
1:30 - 2:00 Walk in place, shake it out
2:00 - 2:30 Step-ups (alternate sides) - 12-18" is usually a good size for step
2:30 - 3:00 Walk in place, shake it out
3:00 - 4:00 Crawl around on all fours - make sure you cross crawl (left leg goes out with right arm etc)
4:00 - 4:20 Burpees (3 max)...if these are too hard, you can see modifications here.
4:20 - 5:00 Walk in place, shake it out
Repeat the above pattern 3 more times.
Note: if you are not "brave" enough to crawl on all fours...hee hee...just jog in place.
A slightly more advanced circuit...might look like this:
0:00 - 1:00 Jog in place
1:00 - 1:30 Jump rope or hop in place
1:30 - 2:00 Walk in place, shake it out
2:00 - 2:30 Step-ups (alternate sides) - 12-18" is usually a good size for step
2:30 - 3:00 Walk in place, shake it out
3:00 - 3:20 Shuttle run...assuming you have enough room...otherwise try this pull-up alternative.
3:20 - 4:00 Walk in place, shake it out
4:00 - 4:20 Burpees (5 max)
4:20 - 5:00 Walk in place, shake it out
Repeat the above pattern 3 more times
An even more advanced circuit...not exactly 20mins...might look like this:
0:00 - 1:00 Jog in place
1:00 - 1:30 Jump rope or hop in place
1:30 - 2:00 Jog in place, shake it out
2:00 - 2:30 Step-ups (alternate sides) - 12-18" is usually a good size for step
2:30 - 3:00 Jog in place, shake it out
3:00 - 3:20 Shuttle run...assuming you have enough room...otherwise do box jumps
3:20 - 4:00 Jog in place, shake it out
4:00 - 4:30 Burpees (how ever many you can do in 30 secs)
4:20 - 5:00 Walk in place, shake it out
5:00 - 5:30 Lunges (whatever style you prefer)
5:30 - 6:00 Jog in place, shake it out
6:00 - 6:15 Pull-ups...if you can't do them, you can find a clever alternative here.
6:15 - 7:00 shake it out
Repeat 3 or 4 times
Getting Creative
If you are particularly resourceful, you can sometimes you find can places to run even though it seems virtually impossible.
For example, a mile or two from where I live is the Capetown Plaza. KMart is there...along with a bunch of other stores. In front of the stores is a sidewalk which is covered by a very substantial overhang. It shelters passers by from the elements. The stretch of overhang runs about 2/10 of a mile. We run there. Down and back is .4mi...better than a track. It gets tedious...so we'll sometimes throw in sets of 20/40's (20secs fast/40 secs slow) or 30/30's to shake things up a bit and help pass the time. But after a while it's really just running. The most I've done there is 94 minutes. You can zip into Panera's for a coffee and a snack afterwords...which can be a nice treat. It doesn't work if there is a southwest wind...but winter storms here more often have a northeast wind so we're usually in luck.
Many times...high schools, colleges, public areas are plowed really quickly...much faster than the roads. You can often carve out little loops where running is possible. The main thing to understand is that you really need very little room. You can do a perfectly good workout in parking lot...as long as the footing is OK and there is no traffic. You might have to invent little patterns or games to pass the time, but once you get past the first 10-15 minutes, you relax and realize you are enjoying making the best of things.
With all these bad weather alternatives it's best to take a "lets just start and see how it goes" approach. Don't set the bar too high. Just give it a shot. If it's not working, bag it. But start...try it out... a few minutes at a time. See what happens. Be OK with looking like a bit of a nut.
It's fun to affirm that nothing can stop you.
And then again...a long winter's nap can be nice too...