Hi! The Montclair Rec Center in Lowry might be a bit far for you, but they have an awesome indoor pool and rooms for parties. I've never hosted one there, but we've been guests of parties held there and my girls (6 and 4) absolutely love it!
Another idea is to have a tea party at your house. I just had one here for my 6 year old's party and it was a lot of fun. You can rent kid sized tables and chairs from Butler Rents or another rental place, then decorate with your own linens and fresh flowers. I also made turkey, cheese and jam sandwiches, then cut them in the shapes of stars, hearts and flowers (using cookie cutters) for lunch. Added some carrot and celery sticks and some cheese puffs...voila, lunch! And Child's Pasty shop in Denver does a really awesome teapot shaped cake. I also ordered teacups and saucers in packs of six (actually demitasse cups) from Ebay for the girls to use at the party (with chocolate milk, not actual tea) and take home with them as favors at the end. It was a really cute party and all the girls were in pretty dresses and on their best behavior! Since your girls are so far apart in age, you could have a big girls' table and a little girls' table...We had lunch, did some classic birthday party games (pin the handle on the teapot, musical chairs), a painting craft project, had cake, opened presents, then the party was over.
Another idea is (if you have a big enough yard) to rent a jumpy castle for the day. Kids really, really love those! You can find a bunch of places that rent them online. They're usually around $150-$200 per day (not cheap, but still less than you'd pay to have a party somewhere else). They come, set it up and when the party's over, you can go in there and jump around with your kids until they come to pick it up. It's really a lot of fun! Anyway, hope this gave you a couple of ideas...good luck and enjoy!!